[Suzerain of Sheol]

That sounds unlucky. And, Amilia is only level 4. She should probably be higher, but then, the fight would be almost too easy since she gets Godspeed soon, which hastes the entire party.

You do have healing items, by the way, though I'm sure you figured that out. It can be tough to use them in battle, though, especially given how many multi-person attacks he has.

And, I didn't decrease the value of the Blood Banquet attack, I just lowered their % chance to use it and lowered their max HP so it wouldn't benefit them as much.