|With plenty of Horses around to ruin a player's day, Micaiah holds one of the more unique skillsets in the game. The ability to hit Horses and Armored Units for effective damage allows her to ruin Horse Emblem teams, even on defense, when used appropriately with the right party members.|
Firstly, your team. Your team members, optimally, will include a Dancer with Blaze Dance. If you're fighting in Arena, and both your and your opponents are using +10 Units, then you'll also need Drive Atk 2. The buffs you get from these members will grant you just enough stats to handle every horse Unit even when the AI controls your team. The +4 Atk from Blaze Dance is just enough attack when combined with the skills to deal with a fully buffed Gronnblade+ Cecilia at +0, and with Drive Atk 2 you'll have just enough to handle a fully buffed Gronnblade+ Cecilia at +10.
Finally, Vantage 3 is a user-friendly skill you can use on your own timing when controlling the team, and while generally irrelevant for Micaiah when AI-controlled (mainly because she either gets one-rounded or one-rounds the opponent), it can be used to great benefit for that last little bit of damage. Just beware of full Ward Armor teams with Vantage, all Blue and Green Units on those teams can survive one Micaiah hit.
Secondly, it's important to use Fury 3 and Atk Ploy 3 as your A and C slots, and Deflect Missile 3 as your S slot. You need the Atk reduction of Atk Ploy to survive a fully buffed Cecilia or Gunnthra wielding Gronnblade+, and with 38 base RES (41 with Fury 3) you have all the RES you need to afflict enemy horses that don't have a RES boon. As for Fury 3, it lets you win the matchup against Cecilia. Distant Def 3 is a nice A skill, but it doesn't let you do enough damage to one-shot Cecilia. You need that added defense with just a bit more damage. Reinhardt is thankfully an afterthought and you one-shot him either way, and against Brave Lyn (both varieties, Brave Bow and Mulagir), Deflect Missile 3 as your Seal will let you survive and hit her hard back. Since DEF and SPD banes for Micaiah don't affect her matchups against Cavalry, it's just more important to have some SPD against non-Cavalry Units.