You can easily cheese klaus first phase by stun locking him at his sack. Due to ranged weapon he does not get close enough to cast any of his spells either so essentialy you are free hitting him.
During second phase you can use sack to stop his jump as well.
He should either not get stun locked with me hitting him or sack should not be permanent object so he can walk or jump through it.
Summon klaus, play Wanda with her new weapon and stand on the other side of sack and free hit him to death.