Win condition: Rout the enemy
There are two bridges/planks leading to the enemy pirate ship – if enemy forces surround you from either side, you're going to have a hard time. To prevent this, send Celica to the entry square of the right-side bridge, blocking the path and protecting your army. Place Genny immediately behind her, and use Invoke to surround her with illusory soldiers (if you don't have Invoke yet, return to Novis Cemetery and grind a bit more with Genny). This way, Genny will be safe to heal Celica, while also attacking from behind protection and healing herself after Invoke using Nosferatu.
Send your remaining units to the left of the map, receiving any enemies that approach. Some of the illusory soldiers will join you, and you should let them weaken enemy pirates before finishing them off using your own units. After taking out the pirates that do cross the bridge, head back to the right of the map to provide support, as Celica may be getting worn down by now, even with support from Genny. Use Mae or Boey to help Celica take out the enemy commander and the mercenary with a leather shield, as magic attacks will be much more effective than physical. If Celica has learned Fire, you can use that as well.
Alternatively, you can also have Celica retreat to the supplies space nearby to recover HP, but make sure Genny has enough illusory soldiers to maintain protection if you do so. When the leather shield wielder is eliminated (feel free to summon more illusory soldiers if needed, but make sure Genny remains out of harm's way and still able to heal Celica and your mages), pursue the fleeing pirates and defeat them, as well as the commander if you haven't already to complete the map.