why not something like this:
def. decrease when near said monster.
monster gives self attack buff.
aura depending on stratum theme (shock, fire, stun if gremlin, curse if fiend etc...)
dodges bullets, but not like normal monsters. this one can stop them in mid air(when facing gunner)
can walk up to and interrupt bomb fuse when facing bomb
can grab swords and hurl swordsman away.
ranged, melee and area attacks.
rare item/material drop or make it always drop a random( or maybe chance of unique drop like luv pup) piece of equipment according to the tier.
explodes when dies(MWAHAHA)
creates ghost versions of self
blends in with the environment
periodically stops to catch its breath, cuz its so friggin awesome. your chance to do most attack dmg is here, but might teleport or spawn copies of itself
looks AWESOME.
not so much hp, and u can also have a chance to make it get distracted by hitting these buttons, each one creates a fake image of a knight, and the monster may or may not attack it.
make it an option either to fight it or go on the elevator, without a prize.
hahahaahahahahahaahahah

^ That's the point, rocket rooom is hard only because there are A LOT of enemies and are too strong, not because of clever AI.