After a failed attempt to convince Xander not to fight, Corrin must face him in a duel.
Defeat the Boss
You can finish this battle simply by defeating Xander. But if you want to defeat all of the enemies in the map, you can kill the General guarding the southern door of the room where you and Xander are fighting, then have someone who knows Locktouch open the southern door. Then Corrin will have to leave the room so Xander will stop attacking.
Then you can proceed to go around the room, killing enemy units. When you are ready, you can go back in and kill Xander.