What could I do to fully utilize a +Atk -Spd Mikoto?

She comes with two really good skills, so I'd say there were two ways to go about building her.

First is using Infantry Rush to ensure your units get the special charge. I'd run Fear+, Windfire Balm (or Kindled-Fire Balm on budget), Infantry Rush and Drive Atk for her seal. This will, optimally, give the enemy -7 to attack and allies +9 (or +7), which means that infantry allies will almost always get the special charge.

The other is keeping her native Flash+ weapon, which really helps your units to initiate safely. You could really easily pair this up with double Savage Blow to help your units ensure kills on the player phase, or build for a defensive buffer if; A) they can't kill the enemy on initiation and B) they have no way of repositioning out, which means they will take a hit. The defensive build might look like this; Flash+, Earthwater Balm+, Spur Def/Res, Spur Def. This, optimally, gives your team +13 defense and +10 resistance, which is pretty huge.

For herself, I'd run her native Brazen, she's pretty good with it. You could run Brazen Atk/Res in her seal slot, if you aren't using it for the team, to give her +14 attack and resistance, which would mean 64 attack if using a 12 Mt weapon. For B skill, if you have the fodder her best set is easily Wrathful or Dazzling, with a refine for the others, but in the absence of those skills WoM for pure support would be great. You can run Dazzling refine for support or Wrathful for less security, but the added ability to act as a sweeper. Those are my thoughts, do with them as you will.