Skills for +Res -Def Micaiah?

Build her the same way you would build Arvis, Deidre, Julia, or Maekoto.

QR or Breaker (depending on your A-Skill) is optimal most of the time but Renewal can be taken to make her a good passive or even a semi-dedicated healer. Distant Defence is a fine A-Skill but Death Blow can also be good if you prefer to focus on her weapon. I don't like Fury since it get's in the way of Sacrifice and QR/Guard/Breakers, especially with her below average HP stat, but it's definitely still viable since Micaiah can make use of every stat. And Ploys are obviously her best C-Skill, but Buffs and Threatens can be taken for a more supportive or budget pick. Finally, DD and Ploy are good Sacred Seals but stat boosters are also amazing for her along with Deflect Missile as a decent pick too.

Hope this helps.