There is a corridor just before the gates that has a chest containing 25 Ice Arrows, guarded by a red ninja. Also, there is a chest to the right just after you enter Soullage's cage that has a longbow, 25 Ice Arrows, and regular arrows. The ice arrows are really the only way to beat Soullage. Hit him with one and he will turn to ice. Run up, attack him a couple of times, then freeze him again. By doing this, he can be defeated without you getting hit one time.