Well, I'd recommend doomwarding as an enhancement.
+1 extra attack per turn, and 1 reroll (of anything - include other people's rolls outside your round) per turn (you burn a charge each time, but with a Fiend of Possession that's a non-issue).
It costs 38500 gp instead of a +number bonus, but FoP is based upon value rather than straight enhancement bonus, so you'll just pick up +4 worth of enchantments and doomwarding, (assuming sixth level FoP),