what boss should not be able to do

ideas about things that bosses shouldnt be able to do.
for example being able to chain stun you long enough that it kills you and you cant do anything about it.

He should be able to do that. But if he can, he can only do it to one person, not everyone at once, and if he does chain-stun that one person, it leaves him unable to hit other people until he finishes the chain.