To defeat the White Witch in the last level, run until she does a backflip. Then, above her head a circle will read "5+". Hit her five times, then a bow and arrow will appear. Shoot her with Susan. Then, hit her with Edmund. She will call giant ogres to attack you. Kill them with the barrels. Repeat, then set the barrels on fire and hit her with them. Repeat the first process, repeat again, then Susan can blow the Ivory Horn to summon Aslan, the king of Narnia. After that, the level will be completed.