The Dark Phontifex and future Builds. Thoughts please!

Maybe try give an example unmerged to give a better picture.

But I think fury works well for him, he'll want to use all of his stats.
He'll want to have the right team though:
Have someone who can heal when they need to.
Buffs are going to be important. A mixed team could be nice.
Fliers and cavaliers are great for synergy with Imhullu.

Turn 3 works nice, it's usually turn 2 or 3 where you start fighting back. So time it well and swoop in with high mobility units.
Maybe use reposition on Gharnef so he can launch someone into battle at the right time.

Res Ploy seems unreliable with only 33 Res.

I would stick with Chill Atk because of Wary Fighter and other high speed units coming out these days. It lets him support the team better too beyond his Turn 3 play. I assume his team mates would appreciate the attack debuff. As for the Res ploy, I've got a Gunnthra who runs Res Ploy with 33 res. I wish she had more res to ploy with...