I highly doubt it matters much, since everyone's voting Sephiran including me, but to be contrarist...
* OK: Yes, I know, arguing with OK interps. Sure, Mantle's regen is the argument for Sephiran, but since I presume you mean "Mantle's nulling..." if you require it to be a specifically Yune-blessed weapon, well, that kind of mocks the idea of the DL. If you require it to be something of equivalent "power" then Alicia is one of the rare duelers who almost certainly qualifies! She's possessed by a goddess, and sort of becomes one by the end! (and surely doesn't overwhelm OK boss HP but whatevs)
* Elf: In the actual case of an eternal match, while I'd normally go for the boss especially since it's possible to screw-up and mis-time Heal, I'd be inclined to tiebreak for Alicia. While we're on the plot-centric voting above, Sephiran won't attack Micaiah, who is another one of those exiled princesses with godly powers / inspiration. (At least directly... I forget if she gets hit by his MT?) He should probably be punished for this chivalry in the DL somehow, where magic princesses, while common, are still less common than a guaranteed member of a ~13 person party.
