My god, I played through "Regret" last night on Co-Op Legendary. It'd have been insanely tough just like that, but I went and picked up the skull you find on the backside of the structure you get on after the first gondola, and it makes all the enemies INVISIBLE. That includes everything, so my friend and I inched through the entire level fighting Legendary grade Grunts, Elites, Drones, Snipers and even Hunters, all of which were invisible. We FINALLY beat it after a few hours, and it was the most satisfied I think I've ever been after beating something that seemed impossibly tough. Part of what made it so incredibly great though, was that after getting owned again and again by one hit kill INVISIBLE snipers, we changed tactics and actually made a little progress before getting owned again, and then there's that one perfect run where we both ALMOST die but we don't and you stick an invisible drone with a plasma nade and it flies into some elites and takes thier shields out and both of us unload an entire SMG clip on them, killing them and giving us THAT BEAUTIFUL CHECKPOINT!
Christ man, great great stuff...