The Ghost King

[Norfleet]

The GK really doesn't play to Arcoscephale's strengths: Priestesses can't remove afflictions for undead units such as the GK, so you'll still have to pursue conventional affliction removal aggressively.

I'd suggest more air, as it improves the defensive effectiveness of Mirror Image, less Water, as water magic is mostly useless past level 2 (Quickness(1), BoW(2)), and more Death. Don't be afraid to take some negative scales to kick loose more points.