Win condition: Rout the enemy
With Valbar your life is about to get a lot easier, assuming you recruited him and have been investing the proper XP (give him a Blessed Ring for HP restoration each turn). Simply lead your army with Valbar, placing him squarely on the bridge leading to the enemy pirate vessel, defending your other units and essentially acting as a wall and human shield.
Press on with Valbar, using your Mages and Celica to finish off pirates he damages with Fire and Thunder from behind. The enemy commander is no special threat, so defeat him with a unit you'd like to score some extra XP for.
The enemy force contains Brigands and Mercenaries, none wielding any particularly threatening items. A few can muster a decent chance of landing a critical hit, though, so be wary with your units who aren’t sporting high defense stats. If for some reason you don't have Valbar or he's been killed, simply lead with Celica, Saber, and Kamui, interchanging them if their HP gets low and using Genny to heal. As always, you're free to summon illusory soldiers with Genny, but in this case you'll likely want the extra XP instead.
Defeat all enemy units to clear the map.