Nightfall becomes criminally hard in the last few missions, best way to deal with it is to level up your lightbringer rank a bit doing quests before you head into the underworld - the damage reduction bonus the title gives is a life saver.
Aye, the Lightbringer certainly helps - the only mission I found was hard was that horrible Gate of Pain. And that was mostly because I could never find a party who knew what they were doing and so we ended up wiping quite early.

