Okay so I've entered the new reaper difficulty to see how things pan out, and let me tell you - this stuff is no cake walk, at 10 skulls so far it would appear that just about ANY Build dies. I don't care how "over-powered" your build is - this difficulty (epic reaper 10 skulls) for devil assault spawns like 20 freaking champion devil mobs who each hit you for 1k damage when 90 PRR (47% damage reduction) is present. Extra mobs.
To beat this difficulty in general, so far it appears you might need a team to win, but once character copy gets fixed, and I can bring my loot over, I might be able to replicate that elven super-tank build and see how it fairs against reaper mobs (TBH, I think they ALL have true-seeing, and they don't bloody miss).