Good Micaiah build?

I have a F!Robin Close Counter build with Gronnwolf. Her defense is 32 with the defense refine and 38 with Close Def. Micaiah will have 24 defense plus buffs

Close counter is a really good skill that IMO only works for few units, and Micaiah is not one of them.

because micaiah has really low def and low spd that most of the physical units that she will encounter she will recieve really heavy damage.

The build is good and if she will be attacked I'm guessing she will survive 1 or 2 rounds of combat against any melee physical unit

Unfortunately Micaiah fits into the Sanaki/Julia mage archetype of being super slow with terrible defense, but really high attack and resistance, which typically make them really good mage counters but will get crushed by a majority of the common physical units out there, so while not completely unusable, I don't believe close counter is the optimal skill to run. She will, however, be pretty good at baiting Nowi/Tiki's in arena if you run into, and CC is one of the highest weighted A slot skills, so if you're aiming for a higher Arena skill without merging its pretty decent.
Normal close counter mages are usually bulkier like Boey with his Gronowl tome (most common), or one of the armored mages that can stack ward buffs, that can take physical or magic damage.
Personally, I run a Desperation+Brash Assault SS combo, get her under 50% with a sacrifice and she can pretty ORKO most red/blue and even a few of the less bulky green threats.

She could be good once she's in vantage range. Most people opt for owl tome effects with close counter. The problem is that, while 58 physical damage from neutral threshold isn't bad, her speed is very.. meh. Odds are that if she can't one shot things, she's gonna die.

But against this ai, she can surely do well enough.

I'd argue she runs it well, actually, but I'd swap close defense for distant defense. Then she can bait and kill most mounted/armored bows and mages (she can easily one shot most builds of brave lyn, for example, before she can get doubled). Then she's in perfect vantage range to take out the rest. With a proper support team (Sothe maybe for a spur + peshkatz buffs) she can decimate a lot of common armor and mounted units.

...that said, she's seriously weak to melee infantry. Units like Mia can eat her alive, close counter or no.... but it's not like she would have survived before CC anyway. :P