The basic idea is to try and put enough pressure on the enemy to, at the very least, knock out at least one of their teammates, and ideally stall them out long enough to where they can't kill Grima.
A few basic tactics:
-There's no way to bait Veronica. She can't be countered and there's no position she can be baited from that won't prompt Micaiah to dance her for another round. This combo will likely get at least one enemy unit killed.
-Because of where the Fortress is placed, there's no position where a unit can bait Reinhardt without also being in range of Micaiah. Rein is equipped with QP Moonbow, so he's difficult to tank without a strong green unit, which will get obliterated by Micaiah. Even neutral units will struggle to tank both of them at once. Alternatively, Rein may survive the bait and get danced for another attack, probably taking out a Red unit in the process. Not something my opponents can afford on this map.
-It's not possible to hit-and-run Micaiah. The two traps placed in front of her are real, so any attempt to snipe her and retreat will get negated.
-And then there's Grima. He's here for the sole purpose of stalling out the match. If opponents manage to reach him, they have to contend with his Wary Fighter+Warding Breath+Glimmer combo, so he'll likely survive even a strong attack and launch a fatal retaliation. Beating the map requires the opponent to survive the initial Rein/Vero/Dance combo and be in strong enough condition and have enough turns to reach and take out Grima.
So, uh, we'll see how this one works out. My strategies have a tendency to be less effective than I imagine.