You need more mixing in your monster groups.

Consider that a group of six Shambling Mounds is a lot like six tanks and no healers. A pair of Guardian Nagas is a lot like two caster-clerics with no tank.

A naga would really like to have something large and beefy standing in front of it to absorb hits while it throws out spells like flamestrike, bestow curse, and banishment. A naga that gets stuck into a melee fight is going to have a bad day, and as highly intelligent creatures, they should be doing all they can to avoid that fate.

Similarly, a shambling mound isn't a good combatant if the PCs don't have a strong incentive to go past it, or something to force them into melee. With no ranged attacks and being blind outside 60 feet, if the PCs can just back off and drop artillery fire on them from a distance, it's not much of a fight. A large part of a shambler's challenge rating is their ability to engulf, effectively removing a PC from the fight, so being able to stay out of Slam range is huge.

I'm not saying every monster group needs to be made up of a tank-artillery-flanker combo, just that those combos can be much stronger than several of the same monster type, and tactics play into that too. Bad tactics can make even a dragon easy to kill.

I'm a little curious about you calling out fireball as a major contributor to the shambling mound fight. Shamblers resist fire, so they should be taking around 14 damage on average from a fireball, or 10% of their total HP, and twice that at maximum (but all those dice mean fireballs tend to be strongly biased towards average damage). That doesn't seem worth of calling out as an important turning point of the battle, at least. Did you maybe overlook all the damage resistances shambling mounds have? Or do you have an Elemental Adept situation going on?

The CR math is based on the assumption of 6-8 encounters per adventuring day

You mention in the comments, in response to my question, that the party takes a lot of rests. The CR math is based on the party having to ration their resources over longer adventuring days with more encounters and a few short rests. Your party is not (necessarily) too strong for their level, they just fight much less than the CR chart suggests.

My personal experience is that for days with a single encounter, the CR chart cannot really be used. Anything below "Deadly" is a cakewalk, and even Deadly encounters will usually have to go considerably over the threshold to be actually difficult for a level 11 party able and willing to expend all their daily resources to win that single encounter.

To rectify the issue, create adventuring days with more encounters between rests. With previous encounters wearing down your PCs' resources, you can expect them to actually start struggling in Hard and Deadly encounters as expected.

Another reason to lengthen the adventuring day is improved class balance: see How does the 'five minute adventuring day' affect class balance?

How to lengthen adventuring days?

There are many approaches, and the one I'm going to present is probably not a good fit for all groups. However, it's the one I have the most personal experience of, and the one that I have liked. Do not implement it without talking it over with your players first.

My method is simply to restrict long rests to known safe areas, such as towns or other established sites. This is, in my case, a house rule, although it doesn't technically have to be one. Restricting long rests forces the players to ration their daily resources until they complete their current quest or tactically retreat --- however, it also necessitates that you plan your adventures in proper size that they can be completed between two long rests.

I've introduced it to my players along these lines:

I have noticed that our game suffers from what is often called the "five minute adventuring day" problem. I have been giving you few and hard encounters per long rest, meaning that you seldom or never have to think about conserving your resources beyond a single fight. It has made the challenges you face hard for me to calibrate, and unbalances the game between classes that have limited but powerful resources like Wizards, and classes that have smaller reserves of power but it's regainable, like Warlocks.

I want to broaden your experience with the game by shifting the enemies you face closer to the recommended adventuring days of the Dungeon Master's Guide. This will mean that individual encounters are much less challenging, but you will face more per long rest. From now on, long rests will be more limited and you will not have the opportunity to rest between every combat --- this is not to make the game harder for you, but to make your tactical choices more interesting and meaningful.

Instead, you will get to take a long rest in safe areas between and sometimes during excursions --- towns, encampments, places like that. Retreating from adventuring to rest in the town is acceptable, but will usually give the bad guys a chance to revisit their defenses or otherwise react. Try not to do that.

This hinges a lot on the players' goodwill, but then again, lots of thing in the game do. In particular, you might need to bar certain spells and abilities such as Leomund's Tiny Hut and Rope Trick (or house rule one cannot take a long rest with them, only a short rest). Banning things without player buy-in is not a good idea, but house rules like this have worked for us, so do give the idea a fair consideration and encourage them to, too.

As others have noted, the CR math is based on your group doing many encounters per day. If you're letting them do only one encounter per day, they're going to be stronger than expected, and in particular your characters that rely on daily powers (like your wizard) will be much much stronger than expected.

But there's another thing going on here, which is that you don't need to serve them fixed-size encounters. Most players in my experience don't need the battles to be super difficult; all they want is for a medium-difficulty battle to go on for long enough that they can show off their awesome moves. So what I do is I give them a battle with a couple of medium-difficulty monsters, and then if they kill those too fast, I start adding reinforcements. Every round another monster or two shows up and joins the battle.

Sometimes these battles are just attrition ("kill the monsters and all the reinforcements") but other times there's some specific objective, like "unearth and open the coffin and kill the vampire who's calling all these reinforcements".

You've also asked about how the group could need to fight multiple battles in one day. The most common reason is time pressure:

• There's a vampire in the village, and it's daytime right now, but if we don't deal with him before sunset, he'll attack us as we try to rest


• There's an army headed toward the city, and if we don't kill all the leaders within three days, it'll hit the city and burn it


• We're raiding the lizardfolk temple, but there are lizardfolk villages scattered all around, and if we stop to rest they'll catch up to us


• We're raiding the Temple of Elemental Evil, and so far we don't think the cultists have noticed we're here, but if we stop to rest they'll restock the guards in all the rooms we just cleared

Good luck with it.

Meaningful Combat vs RPG Style

A lot of the answers to this question focus around the issues with CR and how building encounters with it is more of an art than a science. This is definitely true, but I feel like there is a second side to the coin.

You say that your party is more interested in the RP aspects of the game instead of just combat, and so they are able to effectively long rest before every encounter. This is going to be a big part of why they are able to steamroll whatever you throw at them. A fighter with full health backed up by a wizard with all of his spells is going to be a deadly threat every time. The whole point of multiple small encounters is to make your party think about those resources, because if they blow them all in the first fight that fifth one is going to be tough.

There are groups which really like that sort of tactical decision making, and D&D as a system has given them a wide array of tools to play with. But your group seems to fall on the other side of things, where they want their challenges to be full of narrative meaning. The actual mechanics of the fight are not as important as what that fight means. While D&D might not give you quite as many tools to facilitate this, the system can make it work as long as you don't mind putting in a little more thought and effort.

Moral Challenges

One of my favorite types of encounters to throw at a really strong party is a moral challenge that completely ignores their strengths. You know that your PCs are strong and can take on most threats, so instead of having them fight something that they don't care about (like your Shambling Mound encounter) you need to give your players enemies that they can sympathize with. Make your players actually think about the fact that they are murdering a person and what that means on a moral scale. Some basic examples could be:

• 
  Brainwashed Villagers - I have run this myself with slightly overpowered PCs. All of the villagers in a nearby town have been mindcontrolled by some kind of baddy of your choosing. They are then sent out to fight/delay the PCs from reaching said baddy. The fun part of this is that your PCs will know for sure that these are innocent people. It doesn't matter that they could slaughter the whole village with no problem, they have to figure out if they should, and if not how they handle the fight without killing anyone.


• 
  Sympathetic Bandits - This is honestly a little tired as far as tropes go, but could still be played around with. Your PCs go to investigate rumors of a new group of bandits that have been attacking people on the road, only to discover that they are really just people in a bad spot surviving the only way they know how. Maybe they are deserters from a recent war, who left after being told to commit a war crime. Maybe they are the remains of a village that was wiped out by monsters. The main thing to remember is that you have to make them relatable, and you have to make sure your players know why.


• 
  Wild Animal - It is possible to make non-humanoid creature sympathetic as well. Have your PCs learn about some wild animal that has been attacking people and send them out to investigate. Then let them learn that the animal has been injured or wronged in some way and that it is not just lashing out for no reason. Maybe it had an egg or child stolen by a local hunter, maybe it's den has been invaded by some kind of hazard that has driven it out into the wild, or it has been infected by some local magical pollution and driven mad by it. Make it very clear that the natural order has been upset and that is the cause of the animal attacks. Then the party can debate whether to actually kill the beast, or to try to solve the problem in another way.

Those are some simple examples that are generic enough to fit into pretty much any campaign. In each of them the really challenge your party will face is not the fight itself, but the story around the fight. Should they kill the wild animal even though it is just acting naturally? Are there cases where banditry is acceptable, or should all lawbreakers be held accountable? The focus of these encounters is the roleplaying aspect that happens before/during, and not the fight itself. So it doesn't matter how easy or hard that fight ends up being.

Puzzle Encounters

I won't focus on this as much as the purely narrative fights, but it should still be mentioned. Much as the examples above are ways to tie roleplaying into combat you can also tie puzzle-solving into combat encounters. If your players really enjoy doing puzzles in their dungeon delving then this could be a fun way to give that a little bit of edge.

Essentially what you want to do is make it so that brute strength does not help as much in the fight as thoughtful cunning. You can make the puzzle part of winning the fight, such as having enemies that are only injured by performing a specific action, or you can have a puzzle that needs to be solved during combat. The second one splits the party up a bit, mentally, since some people will be focusing on holding off enemies while others work out the puzzle. Here are a few simple examples:

• 
  Simon Says - The party needs to step on glowing stones on the floor in a specific order. However, the room also has a number of hostile creatures in it which can also trigger the glowing stones, which resets the puzzle. Stepping in the right order while maneuvering the enemies becomes the puzzle.


• 
  Elemental Chaos - The party encounters a large room filled with small elemental spirits. These should have their stats tweaked to be extra resistant to non-elemental attacks but extra weak against their opposite element (e.g fire vs water). The elementals can be tricked into hitting each other, or the wizard can use the appropriate spells. For added fun you can have the elementals "die" from normal damage but reform after a few turns. The puzzle here is finding the right way to permanently disable them.

These fights are a lot more enjoyable for parties that don't mind combat but also like to do problem solving. They also make it so that the party being overpowered isn't as big of a deal because winning the fight is only half of what they need to do. Something like the Simon Says encounter can still be brute forced by killing all of the enemies first, but it at least adds some flavor to the fight that wouldn't normally be there.

The important thing to remember is that D&D encounters don't have to be purely mechanical. If your players don't find that fun then figure out what they do like and focus on that instead. You could easily run some of these examples and end up with the PCs not fighting at all, but still enjoying every minute of that session. As long as everyone has fun, play the game however you want.

I'm gonna try to shy away from the CR and purely mechanical aspect of balancing encounters and maybe tackle different aspects of what I read between the lines in this question.

As I understand, here are parts of your problem

1. Fireball is used so effectively that you felt the need to nerf it. You're not the only GM to have this issue and I do not remember a GM ever nerfing that spell in my groups. Maybe there are alternative solutions ?


2. Polymorph is also mentioned and I'm guessing you are having issues with what the wizard is turning itself into being possibly OP ?


3. Cookie-cutter encounters are just that. You could build more challenging encounters by building them yourself while keeping your group's capacities in mind.


4. If anything, from the list of magical item you mention, they are under powered for their level !! I'll try to tackle why they can still seem over powered while they mechanically are not, in my 'barely informed' opinion.

Fireball is about more than doing damage

It is an explosion. Fireball is not always safe to cast. It can backfire, too.

Fireball is a very popular spell. I don't have real statistics, but I'm sure 95% of players who can take it do take it. I've learned to deal with this spell when it gets abused and becomes the de-facto answer to any and all challenges I throw at my players.

Maybe casting a fireball inside of ruins is not safe ? It can cause passageways and roofs to collapse, possibly even ending the encounter before it starts by trapping the group, possibly even splitting them up ? For the sake of "fun" and to allow the dices to tell their story, I would not make this something that would automatically happen, rather give your spell caster a chance to notice that liberal uses of fireball might have consequences, besides efficiently killing mobs. If he keeps casting fireball, make the chances of a structural collapse more likely every time, until the group pays the price !

They are not in ruins or mines or caves than can collapse on their heads ? They might be in town where one fireball can set fire to an entire medieval city ? Or in a forest where carelessly burning trees might gather some most unwanted attention from the guardians of the forest ?

Fireball can also destroy items. That's also loot getting lost ! After 1-2 such occurences, I guarantee your other players will be the one trying to keep the wizard from using his fireballs.

You shouldn't be too heavy handed since what you want to achieve is for your spellcaster to be more careful, not have him stop using a spell that is legit.

Also, creatures with resistance to fire laugh at fireballs. Or your mobs might be buffed by spells that allow them to resist fire.

TL;DR Nerfing the spell is too heavy handed to my liking. Instead, I would make sure my players realize their are consequences to causing explosions. Also, there are spells and resistances that can be used to give an edge against fire damage.

Any problems with Polymorph ?

I'm gonna take a wild guess and assume your Polymorpher transforms himself into a T-Rex or some other very powerful but also very rare creature ?

As a GM, I often tackled this issue before it even could become a problem, with characters that I know will get access to that spell or any other kind of shape shifting abilities.

I make sure to make it clear to them that they do not know and have not encountered all creatures that exist in the world. I specifically rule the T-Rex out of the possibilities, unless the character can really convince me that it would make sense with a good story.

I'm actually pretty permissive with polymorph, most of the time only requiring the character to succeed on a Nature check to see if I allow him to transform into this form. I often adjust the DC of the check according to the CR of the creature. But then again, it has to be believable ...

I've had players able to transform into a T-Rex at lvl 7, as soon as they could. This is OP ... but Polymorph is also a Concentration spell, which is supposed to balance the power of the spell, so do not forget about having him roll concentration every time he gets hit ... the next time he loses concentration as a T-Rex (or any other powerful beast) and ends up as a wizard in melee range of a lot of enemies that absolutely will proceed to focus him (scared that he would transform into a scary monster again) I figure that's got to make him more careful in the future ?

Cookie cutter might not cut it

I also use random encounter generators, but only for inspiration. Most random encounters generators give you stat blocks, but an encounter is more than just a bunch of creatures fighting.

All random encounters happen somewhere. A lot of times, that somewhere is a place that is unknown to the group (they're just passing through, they don't live here !) but very well known to the mobs you want to put on your group's path.

(Note: All of the following suggestions should be appended with the condition "If that makes sense". I wont repeat it for brevity's sake).

Why wouldn't you add a few traps that the mobs would use to soften up any one unfortunate enough to pass through their territory ?

Maybe the creatures have some means to turn themselves invisible and instead of engaging the group in a head-on, fair fight, they would instead use guerilla warfare tactics to hit the group from multiple positions, ensuring that they cannot efficiently focus everything. Have your mobs disengage as soon as they can after being engaged so that they can kite your players.

If you play your cards right, aka if your mobs have some kind of advantage (preparation, terrain, numbers, etc.) even low level mobs can become a problem for a high level group.

How can my group be under powered on paper, but still breeze through all encounters that are supposed to challenge them, according to the CR mechanic ?

Like I said above, the list of magical items you showed us really is under powered for a level 7-11 group !!! At least, if that's all they have, but you said it yourself that you don't think you gave them too much and I readily agree with you.

So how is it possible that a group that is supposed to be under powered according to the DMG be over powered in reality ? At level 11, each character should normally have more than what you describe (see page 38 of the DMG for a nice little table, even in a low fantasy setting). Disclaimer: keep in mind I don't know the real extent of your group's wealth.

Having said that, I'm thinking the issue here isn't about the spells or magic items they use.

Maybe your players are just very good tactical minds and their characters also are, giving them such an edge that they aren't challenged ? Maybe they 'meta game' because they know all weaknesses of all creatures in the Monster Manual and other source books ?

There are a lot of mechanics you can use to introduce more challenges in your encounters.

Surprise might not work since they might be "that good".

Or maybe you can force them to make knowledge checks (Nature, Arcana and Religion all can give some insights to different types of monsters) before using OOC knowledge to give themselves an edge in those encounters ?

Worst comes to worst, boost some of your mobs, simply.

Victory is fun. But easy victories get old pretty quickly for a lot of people. I personally get bored if I never have to think outside the box to come up with solutions to challenges, so if all I need to do is roll dice to succeed I end up yawning and thinking about other funnier games I had in the past. That's just me, but I figure there are good chances you have players in your group that would agree with me on this.

What to do then ? Well, as a GM I often boosted my mobs on the fly. You don't need to give them access to fireball or polymorph, but a +2 to AC and/or to attack rolls can make a big difference in the damages a group will take even when they win.

Then again, you can surprise them with more than one encounter in a row. There are ways for you to narratively ensure your group cannot take a short or long rest before their next fight. Maybe that first encounter was easy because the mobs were not prepared for a group of high level PCs, but now that they know and have been alerted they might become a danger to the group's survival.

In conclusion: I hope you will find some of this advice useful. I personally constantly use those, and more, to keep my players on their toes. I make sure that they realize they are not fighting "stats blocks", they are fighting monsters who might sometimes surprise them with their behaviours and tactics...

5E encounter building rules are not very good. They are complex, specific and inaccurate.

The damage budgets of monsters (HP and their DPR) go up linearly with level, meanwhile the CR "encounter budget" doubles every handful of levels. Spells go super-linear, melee damage doesn't.

Second, two Guardian Nagas at level 11 is a hard encounter; don't round up. Three is deadly. (I used Kobold)

A "deadly" encounter is an encounter where a player would be at risk of death if the party had previously gone through a half-dozen normal encounters in the day and had 1 or 2 short rests.

That encounter with a fresh party with full resources is going to force burning daily resources, but without bad luck won't threaten killing one of them.

Throw in the fact that the system is very crude and it not threatening the party isn't unreasonable.

You currently have a 5 minute adventuring day. 5e is well designed for 6-8 encounters/day, especially at your level. Many people don't play it at that pace.

You can rework what a rest is if you want to fix that.

A short rest now becomes a night's sleep.

A long rest is now a period of no stress, in civilization, for a week.

This advice already exists in the DMG. It is called "gritty realism". But, in my experience, it is more about combat frequency pacing than anything else.

You have a game style where 2 encounters/day is rare. Match that to your rest mechanic.

I run random encounter check every time my party rests, with increasingly higher chances the more they are in a dangerous area.

for example, a party doing a short rest after every encounter in the goblin fortress, will likely never recover anything since they will keep being disturbed by wandering monsters.

Allowing too many easily done short rest between combat will create that sort of issue.

But it is also OK for a specific encounter for which the party is well prepared to easily dispatch it. (knowing is half the battle). In you example the encounter is not a deadly one since your party had all the proper spells against that monster (fireball against shambling mound), but it is very unlikely that their fireball will be very usefull during the next encounter. Make sure you mix encounters so that it tests their abilities to adapt and also do not allow too many short rests in dangerous area.