Game is poorly optimised for Co-Op

I played with a friend co-op Act 1 on Normal and the game crashed repeatedly after getting beaten around four times by praetor Draeven.

We had freezes and crashes after like the second wipe and we had to alt+f4 twice, until we finally beaten the guy.

i've played with 6 man party for a long time,only about 3 times crash.
it's perfect if you crashed hundreds times at last season.