I basically just focus my team on one enemy at a time so that, hopefully, each turn I take out one of them. That's one less to hit back at us. I use any buff that comes up and shield and heal carefully so that I don't over heal and waste any. I Hope this helps.
If I remember correctly, you first need to extinguish the Fires (this gets rid of the duck) and then you go around killing the bad guys. Don't attack the Duck except area-of-effect attacks because he's invulnerable at this point in the game. Otherwise, Chrissys's suggestions are great and will definitely work. Also, you probably need a few Rouses to heal your characters.