|Micaiah: Summer's Dawn has a fantastic Res stat, a solid Atk stat, and a prf that not only has dual effectiveness against half the unit types but also increases her Atk to boot. This gives her all the attributes to be one of the most powerful support units in the game, able to run dual ploys while still able to pull her own weight in combat.
Fury 3 is the cornerstone of this build, as with a Res boon she reaches a massive base 40 Res, enough to ploy all of the unboosted units in the game. As an added bonus she even gets a decent boost to Atk in addition to her prf's Atk bonus, which in combination with her effective damage means that she can realistically OHKO many of the most threatening armors in the game, such as Zelgius.
Your choice of B skill comes down to how often you expect to use Micaiah in combat: Wings of Mercy is a more support-oriented option that lets her warp to injured allies while Escape Route can be used after 3 rounds of combat, which becomes especially useful in gauntlet style modes since she'll be free to warp to whomever she pleases after enough battles. Ultimately the choice comes down to preference and playstyle and are key skills to any dancer. The new dance B passives are also an option if stat bonuses are desired for her allies. Alternatively one can opt to use a Chill skill to complement the ploy abilities if warp dancing isn't desired.
Finally the C slots are reserved for ploys, which as stated above are almost guaranteed to activate given the Res stacking in this build. Which ploys Micaiah should equip come entirely down to preference or need: Atk and Spd Ploy are good general purpose ploys that always benefit the team as a whole while Res and Def are more specific to whether Micaiah wants to help herself secure more kills or if she wants to help her teammates more. The ploys used can be tailored for whatever the player needs and I would encourage as many ploys to be inherited for maximum flexibility.