Win condition: Rout the enemy
Saber now holds Celica's Gold Dagger – you can leave it with him, or return it to her if you want access to Combat Arts you've unlocked.
To start, cross the bridge/plank leading with a strong unit like Celica, allowing Mae, Boey, and Genny to attack from behind. Once the bridge is cleared, Saber can catch up and clear some units to rack up experience. The enemy will likely begin to retreat – if you want Saber to catch up to the rest of your army level-wise, have him defeat the enemy commander for a hefty experience boost. Otherwise, finish the commander how you see fit to complete the map.