I highly recommend having a character who can steal (Fargo/Kid/Mel) in your party from this point on. Dragon bosses have a nice item you can steal, and as a bonus also have more powerful elements you can trap.

As for Miguel, my strategy was pile on the dark elemental spells to keep the field element as black as possible. Also as his light spells tend to shred me, so do Lynx and Harle's dark spells cause high damage to him.