which Micaiah?

If you aren't trying to make her double, then just throw away her spd. She's going to get doubled anyway.

Well, you can go two different directions with this. You can either go with +Spd and give her Swift Sparrow/Desperation plus buffs to make her double, or you can dump spd and go with the +Atk and give her Deathblow 3/4, Desperation and Brash Assault seal to make her double. I would probably go with the +Atk if it were me, but maybe depending on fodder availability for you might sway you one way or the other

I don't like dumping spd even in BA+desperation because you'll double through wary fighters using the spd and I would expect doubling to get the kill even without 3 extra atk.

making even a +spd micaiah double would require quite a bit of investment and you’d have to (most likely) rely heavily on spd buffs, so unless you’re willing to do that i’d suggest +atk! my micaiah is also +atk (-def) and her attack stat is big enough that she only needs to res debuff her foes to one-shot most of them, usually with ploy skills.

still, a micaiah that can double would be a very unique (and terrifying) unit, so if you have the resources why not go for it?