My friends and I have been playing Dungeon Defenders on PC for a few weeks now. We have a variety of characters of at least level 70, usually focused on either towers or DPS (few hybrid builds in our group). We've cleared the campaign on Insane difficulty (with a fair bit of trial and error, but we did it), but we can't even beat the first wave of the first map on Nightmare.
Any tips? Are there stats we should be focusing on, such as resistances? Items we should look out for?