-Bard songs are definitely effected by damage reduction
-Dispel magic does not check saves. Your sanctuary was never dispelled, because an invoker can't dispel communes.
-It seems pretty obvious that the bard has invoker shields, which make you immune to that damage type
-I'd be shocked if the Watcher started that fight anywhere near full health

And some things to consider:
-Why not kill the nightwalker first?
-Felar chief is a good start, but there's no limit on the amount of stuff you can summon. Raiding as a bard should be impossible, if you have time to prepare--especially summons that will spam lagging moves.
-The warrior had far less damage reduction than the others. Drum spam would have forced him out of the fight (unless he can just redirect and slaughter you...)