Win condition: Rout the enemy
Here you're facing an entire shipful of magic-wielders, so the leading with Valbar strategy isn't going to work (his magic Res is pitifully low).
Instead, lead with Genny across the right side bridge, and summon illusory soldiers with Invoke. Send one Mage to follow up behind her to pepper enemies from afar, while Genny herself can use Nosferatu and her high Res stat to get by (not to mention defense using her summons).
Use another Mage or other ranged unit to pepper enemies across the bow from your ship to theirs. If you'd like, you can also send Celica and a small force (perhaps Kamui or Saber) over to the left bridge to approach and surround the enemy. This is particularly viable if Celica already knows Recover (to heal allies), or has the Recovery passive skill to keep her own HP high.
Continue with this strategy until all enemies have been defeated. In general, be careful of Mire, which has longer range than Miasma. The enemy commander is a bit more fearsome than his troops, but a double-team from your best Mage and Celica or a melee unit ought to do the trick. For the most part, Valbar should sit this one out.