|The Dark Pontifex sports a usable but overall mediocre balanced statline, and so his plans for world domination instead revolve around his unique tome Imhullu, which damages and cripples physical foes on turn three. As such, Gharnef has two turns of preparation before he has to make the magic happen.
Fury forms the core of this build; the fourth-level version is ideal, and does exactly enough recoil damage to push him into Brazen range, until merges or flowers give him 41 base HP. The more accessible third level version can also work, however. The skill gives him decent defensive stats of 30 Def and 29 Res, and a base Spd of 38. Ardent Sacrifice can be used as an alternative method of lowering health, especially helpful in Aether Raids where at least one turn of approach is often necessary. This assist will also guarantee him Brazen range in the face of Duma's unique skill Upheaval.
As mentioned above, he uses his recoil damage to power himself with the seal Brazen Atk/Res. The Atk/Def variant functions about as well here, since the Atk is the primary focus and his gimmick is preventing counterattacks. As archers but not mages are effected by his no-counter effect, the Res variant is favored slightly. If a Brazen Atk/Spd seal ever comes out, that will function ideally. This helps patch up his middling attack stat.
Since he focuses most on his third turn, Odd Wave skills synergize perfectly for him. In lack of a Brazen Atk/Spd seal, Odd Spd Wave is the C-slot of choice to boost up his doubling potential to a more respectable level. If that seal ever does arrive, opting instead for the Atk variant would be preferable. Alternatively, if an ally can provide Spd support, more power is going to aid him more and the latter option is again superior.
His remaining slots are his most flexible. For B, his packaged option of Chill Atk functions respectably, as it is an additional punch to enemy units, fitting in well with his tome. More ideal, however, would the Res-debuffing variant of this skill. Moonbow is his special of choice, given his somewhat disappointing base attack. Given that he uses Brazen Atk/Res in his seal slot, Iceberg could be gambled on instead, hoping for an activation on the third turn (swap out for Bonfire if using Atk/Def). As he has two turns to charge special, various other options could be explored, including AOE, but these are going to be inconsistent in their application and so are not recommended.
Though this build is designed to allow Gharnef to punch through his enemies on the third turn with doubles, it can be subverted effectively with no required change into a support build, as he instead sends his underlings to pick off the weakened enemies. Ardent Sacrifice is a rudimentary heal, Chill Atk or Res is a debuff and so benefit the rest of his team as well, and Odd Wave skills work similarly but as buffs. His decent base defenses and choice of either a Def or Res boosting Brazen skill let him tank a hit or two with reasonable potential to avoid doubles. Don't think of him exclusively as an offensive or supportive unit, and instead of use him as either depending on how the match plays out.