Jtdcftul

After posted this question, it turned out that mimics might be weaker than I though first.

As said in this linked question, I really like mimics and I'd like to use "a lot" of these (without going too far and make my players paranoiac) even when party level is increasing.

Also in the comments of the linked question's accepted answer, Erik told me that:

a basic Mimic probably won't ever challenge a high level party, but there are similar monsters of higher power levels, or you can make your own...

So I now have another question: How can I make mimics a real challenge for a high level party? Which monster would be similar to what a higher level mimic would be? (a party of 4/5 players)

I'd like mimic (or similar monsters) to be a challenging fight, maybe not deadly, but at least interesting fight, not over in less than two rounds.
If necessary or easier, you can split the answer to address to floor level (i.e.: level 5 to 10, 10 to 15, 15 to 20, 20+).

As I'm very new to D&D (only started to buy basic rules, read linked question and this one for context) and even if I read a lot of this RPG stack Dnd questions, I'm not very aware of how to make changes while leave game balanced. So please be clear about how you managed to make it fit to another level (if so).

It also doesn't really matter if it's not RAW as long as it isn't game breaking.

Answers have to be based on prior experience about using mimics at higher level or at least using monsters at higher level than their basic one. If the answer is based on the use of these similar monsters, please back it with some in game experience about it, as a player or a GM, to support the fact that it's similar to a mimic.

Putting monsters in advantageous territory and playing them intelligently can drastically increase their difficulty.

Example from DnD history, Tucker's Kobolds.

I would imagine a couple of mimics in a low-light room filled with regular traps and objects could be devastating to a party. Imagine, a mimic attacking, losing grip on an enemy after doing some damage, and suddenly disappearing into the chaos of traps and objects littered around a room, ready to surprise their prey again.

My strategy is usually to find a monster of the appropriate CR that's roughly similar to the one I want to scale up, and then replace parts of the original monster's stat block until it looks like a challenge. Things like HP, AC, to-hit and damage, save DCs, skill proficiencies and saving throws, can all be copied over. The PCs generally aren't privvy to most of those stats anyway, so as long as you re-flavor appropriately, this kind of direct substitution works very well.

Let's say we want a CR 5 Mimic. A pretty similar monster would be the Bulette. It's also a Monstrosity, it doesn't have any weird abilities like poison or spellcasting that would throw off the CR, and its stats are roughly similar.

So I would then take the AC, HP, ability scores from the Bulette and paste them onto the Mimic. I would keep the abilities and Actions from the Mimic, but replace the Bite attack with the Bulette's Bite - and maybe give the Pseudopod attack a 10ft reach. Maybe you make it Large, so that it can mimic bigger objects, like a statue or wall instead of just a treasure chest or box. And now your mimic is now tougher and does more damage, but still feels like a Mimic.

While I don't have experience scaling a monster up, I do have experience scaling a monster down, and I suspect the same methods should work going the other way.

Basically, what I do is figure out what kind of damage monsters at an appropriate difficulty can dish out, and what kind of health and AC they have, and I give the monster I'm trying to scale similar numbers. While I haven't had a good reason (yet) to scale a monster up, it worked pretty well for scaling a monster down.

I recently surrounded my party (all level 3) with eight Snow Wolves (note that I haven't been able to really challenge them yet and I was looking for that line between "challenging" and "unfair"). With their normal stat lines, these are each a challenge rating of 3. Now, challenge rating isn't a perfect guide for an encounter's difficulty, but it's also still useful. And eight challenge rating three monsters are an entirely unfair fight for a party full of level 3 characters... so I cut their hp in half and dropped their AOE ice cone ability. This still left their damage output higher than the creatures they'd been fighting, and their health was still pretty high, but no longer insurmountable.

The result was that the party was fighting for their lives from the very start of that fight, but were not facing an insurmountable foe. Basically, I got exactly the fight I wanted out of my party and more or less figured out where that line was between a difficult fight and an unfair DM.

For your mimics, you could apply this in the other direction. Instead of using its base stats, take a look at creatures in the difficulty range appropriate for your party. Give it stats like those. Buff its health, make its damage comparable, maybe improve its AC or give it elemental resistances depending on where it's found. Basically, figure out what monster would be a challenge for them, and make its stats like that.

Alternatively, you could wait until after a big fight, and then mix some regular mimics in amongst the real treasure in a treasure room. The party's resources will be drained, leaving them fewer options for how to fight, although an individual mimic still shouldn't be a challenge at this point. So... have them all get up and start fighting as soon as one of them is interacted with. By spreading them throughout the room and around the party, you get around their low mobility, and by sending multiple mimics at them after a more difficult fight, you still make them dangerous to the party. And by mixing them in with actual treasure chests, you make the party second-guess every single chest in that room, making them suspicious of everything in that room!

But... only use that last option if you're fine with your party never trusting a treasure room ever again. They will not forget.