Can CR be used to estimate difficulty for a group of players using monster stats?

Question

I have a situation planned where the players will playing as monsters, using the stats in the monster manual (entirely in lieu of their own). Are there any guidelines anywhere about how to balance encounters between monster groups in this manner?

Note 1: This is not the same question as level/CR transparency; during their time as monsters they will all be monsters, and encounter no 'heroes' with character levels, so I don't care how PC monsters balance against other PC's, only how I can balance them against other monsters.

As a rough rubric, I assume that working out the CR of the PC-monsters, then preparing an encounter with that CR would be roughly "balanced", as in either side is roughly equally likely to win... but that's not how the game is played, the players usually win encounters (with some degree of risk and attrition), and have an advantage in the forms of abilities and options available to them, I'm just not sure how to adjust for the lack of those...

Note 2: I'm hoping to allow players to pick their monster forms freely (or at least restricted by a maximum CR), and have them face a variety of challengers - I'm aware this will certainly result in situations requiring my adjudication regardless of balancing rules used (abilities like a lycanthropes resistance to normal weapons for example will be very strong vs many monsters), but I'm hoping to minimize this as much as possible going in; I am ok with heavy-handing things where required, but would prefer to keep that to edge cases rather than constant moderation...

I'm open to official rulings I may have missed (I'm sure there are rules I've read but not absorbed in the core rule books, and I don't keep up with the living errata on twitter), or any third party/homebrew suggestions on how to balance this.

Answer 1

The way I would go about this would be to use the XP thresholds in the DMG pg. 82. I know this is for PCs vs monsters, rather than monsters vs monsters, but this could be used to determine both sides of the fight without involving any PCs.

First I'd work out your "monsters party's" difficulty rating by trying to figure out how many PCs at whatever level it would take for the monsters to be considered a Medium encounter. So for example, if they were all CR 1/2 Monsters with 100 xp each, that x2 (because there are between 3-6 of them) would bring it up to 800 (assuming you have 4 players). 4 4th level PCs would be able to handle up to 1000, so let's say they're Medium for a 4th level party.

Then, come up with other collections of monsters intended to be the opponents to your "monster party" using a party of 4th level PCs as your benchmark. So if you gave them a CR 5 creature with 1800xp to fight, that'd be a Hard encounter for them.

My reasoning for making the "monster party" equivalent to a Medium encounter for a PC party is so that enemy's CR would be relative to the different categories; an encounter that added up to less CR (i.e. within the "Easy" band) thus would be an easier encounter for this "monster party", whereas more CR (i.e. within the "Hard" band) would be harder, etc. If you want to tip the odds in their favour, find which level PCs would make them a Hard encounter, which makes them stronger relatively, or making them a Easy encounter to make them weaker.

Note that I haven't actually tested this, this is simply a suggestion based on what we are provided with from the DMG, albeit using it in a non-standard way.