Win condition: Rout the enemy
This will be a challenge, especially if your army’s rank is below the enemy’s. Basically you’re facing off against a bunch of super agile Arcanists, many of whom can heal each other, and some of whom can attack from extended range with Mire. If you have a powerful Sniper you could be in good shape, but otherwise, prepare for a grueling fight.
Most of your units are likely pretty vulnerable to magic attacks, the exception being Silque with her great Res stat. You can pretty much send her alone into the fray of foes safely (just watch out for Mire), and she’ll slowly chip away at almost every unit with Nosferatu, taking little damage and keeping herself healthy in the process. Keep your own army out of enemy range for now, and let Silque work her magic.
Once several enemy units have been whittled down, have Silque draw them closer to your main army and swoop in with Pegasus Knights or Alm to finish them off. Archers and other ranged attackers will also be quite useful here, and any time you get in a pinch simply retreat and let Silque stall some more. Regroup, and repeat.
Once most of the Arcanists have been defeated, there will probably be one with Mire still waiting towards the top of the map, as well as the enemy Commander, also wielding Mire. Silque is less indestructible here, so use multiple strong units to take out the first Arcanist in a single turn while staying out of the Commander’s range.
With that done, close in on the Commander with your most capable units to finish him, clearing the map. Make sure the units you choose will survive at least one turn if their attacks miss – even a single mistake can be deadly. Perhaps, like in my case, you’ll get lucky and score a critical hit.