I beat the entire game on Insane with a friend. I thought it was easier with a friend because you can usually help each other out whenever someone is in trouble and the other just cleared his area. The situation happened to me where I couldn't get past a part like that on Co-op on Hardcore but I think that might have been because my friend was new to the game because a little while later I did it easily in one try.

the game is easier on your own but its alot more fun with friends, i got bored really quiicky on the second act onb casual but we're now on the third level on hardcore

INsane is easier with friends on parts where you are together and could heal each other. ON the parts where you are split up like the beginning of Act 4, I really would have preferred doing that with the CPU instead.