For those who couldn't figure out what the hell is going on with Papus, like myself for the first 10 deaths...
The only real way to kill him is to put him in the stun lock. If you didn't pick up any of those skills, you need to.
Papus' range attack can 3 shot you, if you let him get the cast off its pretty much over. Stun, knock back, whatever, as long as he doesn't get to cast, you are fine.
There was some really weird laggy *** going on if you are just waiting to interrupt or dodge, etc. I literally died 5 times when he wasn't even doing anything just stood there. In reality, I am sure the game code has him casting. The only way I can ensure I don't get hit by stuff I can't see is to keep him stunned.