Xeviat
Dungeon Mistress, she/her
Seems like you'll want something like 4E. 1 monster per PC of equal level is a standard fight. Since the 4 players vs copies of themselves should, by its nature, be very deadly, a monster of level X would need to be weaker than a PC of level X.Keep me informed. I definitely want a monster building system that uses character levels and eliminates CR.
The trick is, they need an extraordinarily high amount of hit points to survive a round of attacks from player characters, but cant deal too much damage or they will one-shot a player character.
But it is still possible to think in terms of character levels.
Then you have elite monsters that have twice the HP and more attacks, and you have solos that have 4 or 5 times the HP and have more area attacks or round by round damage.