Bonemass is probably the best boss in the game currently.

It isn't too easy to cheese and can be a treat.
4rth boss is so bugged it's easier than the first, and first is easy as hell...

Haven't fought the 5th, but sounds like its bugged too, so, yeah :/

Bonemass is the only boss that fights back, and not just waits to die.. even so he is completely helpless in coop.

It's freaking sad when a 0 star goblin is more challenging than a dragon boss...
Not even talking about 2 stars...