I've played a few hours of Insane and am almost done with Act 3 with my pal.
We'll have it done pretty soon. Hardest part is by far the wretches that explode.
On hardcore I just sawed the mutherfuckers open.
Their explosion can one-shot you in insane. Plus they take about two melee hits to down you and they attack and move very fast and sparatically.
Trying to chainsaw a group of exploding wretches is 100% death. Even if you saw the first one without being hit, chances are the explosion will kill you. If it doesnt, a second one will hit you before you get away and you're done then for sure.