Nightmare you'll need a full spirit mage w/ revive, i did it with a KE who constantly has barriers and can revive everyone to full health and barrier + a spirit mage who pops barriers and revives also... so its pretty much impossible to die. Blackwall is the best tank in the game, aslong as he has guard he cant die, and he always has guard, Cassandras good too, i chose to go with her because Nightmare you'll wanto focus on defense but try and squeeze out as much dps from everyone as possible, and Cassandras a good tank with some sweet aoe templar moves