My advice? Mystic Knight, magic cannon spam and holy affinity. Put 3 or 4 magic cannon sigils, then attack it, each attack will blast 12 projectiles.
.... I'm not sure why I never thought of that.
I've been using Magick Archer to spam the hell out of Hunter's Bolt. It's slow going, but it's very easy to tell which spots are still active and you hit 10 spots at once.
Anywho, I've noticed that my Sorcerer pawns tend to be more effective against the Ur Dragon than I am. My Pawn will sit there and spam Bolide all day long against him and it does wonders. Thinking about going in with a whole Sorcerer party at some point.