Hey Muddy,
So it's been a little bit since I've played through to the final boss, but he's very-much an attrition fight. He should only be spawning four times and will die for good on the fourth. His moveset will also vary depending on which campaign you're playing.
Big, one-two punch combos don't really work on him. I'd keep away from ice field magic. Maybe forest as well. Fire was always my favorite anyhow.
But really, the thing that always saves me is timing my Ash* cards just right to knock out his big bursts of field magic. If you can avoid a lightning storm or a fire field, you'll be much closer to finishing him off as those spells are painful.
*I think it's Ash cards. The ones that will negate magic on the playing field. You should have at least one by the end, but they are a craftable card. I want to say I went into that fight with two.
Good luck!