Distant Counter

Wouldn't Vantage be the best B skill for Distant Counter Ayra?

If you do intend to give it to Ayra, you want to ditch her namesake blade for Slaying Edge+, and she demands Vantage with that.I would also recommend in that case to give her the Distant Defense seal, because while her Res is pretty good, it's not exceptional.

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I'll let other people comment on your Ayra idea since I don't have her. Some units that I think use DC very well are Myrrh (Iote's shield seal makes her even better), slaying axe + vengeful fighter Sheena, wary fighter + Berkut's lance (res refine) Effie, anybody with strong mixed defenses. I could also see it working with a light breath dragon.

Also fun are WT Reinhardt (especially with that vantage; maybe put moonbow because it's quite likely to activate on one of the two hits) and draconic aura Ares, also with vantage.

Some other candidates: Raven with the same idea as those two, Zephiel or W. Chrom with wary fighter (W.Chrom does especially well with slaying axe + glimmer), W. Lissa, I've also seen Oboro and Roy turned into incredible tanks, and Roy can have QR effect down to 50% HP on his weapon.

Slaying Edge + Wrath is broken, and incredibly fun. Vantage is also good if you're running Slaying Edge. Her default Desperation is also ok, it works very well with her default weapon.

As for other units, it really depends on what you've got. But my suggestion for that is, save it for a merge project, since it's one the rarest and most useful skills in the game, and also the highest scoring A skill for arena.