I find that using a Noble with a shotgun pumped up with damage lv 3 mods is full of win. Just make sure somebody in your group has Engineer 10. Also, upgrade the Villa and party every day for even more damage. I'm on day 151, and doing over 800 damage/shell (that's up to 3200/shot). Get two noble mercenaries and give them gravity guns, too.
To beat Ragnarok easily, get a Chalice of Jure to turn your castle invulnerable for a few seconds, then pelt them with grenades. You'll get lots of EXP out of it.