After overwhelming Camilla, you use a secret passage to reach the castle but find yourself in the middle of a troop of soldiers.
Defeat the Boss
In this circular arena, there are four Dragon Veins that you can activate to create wind storms in the corners of the map. These will help slow down the flying reinforcements that show up throughout the battle.
There are Fire Orbs in the south and the northeast and northwest of the maps that you can use with a tome/scroll-wielding unit.
Four enemies in this map will drop items when defeated.
The flying reinforcements will keep coming until the battle is over, so when you are done earning EXP from them, go north and kill Hans.