How To Beat Seymour?

So I've been stuck on the final boss for some time. My current top team of Galahad, Astraroth, Orion, Dorian and Helios plus Fenris have a combined strength of 800k but I cant get more than 1/4 of Seymours strength down. I've swapped out Dorian for Cornelius but it's a worse result.

Is there a specific team that I need? If I knew I could direct my resources towards those heros and pet.

TY