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Head south, east, south, west, and enter the cave. TheArrow should now be for sale. Buy it. The bow can now be used. Instead of an arrow counter, the bow uses Rupees. Despite the cost involved, it's a powerful weapon. It can kill many enemies in one hit, including the previously undefeatable Zoras, and deals great damage against certain bosses.

Head east, north, north, north, and east. Bomb the far right part of the wall, just before the corner. Enter the dungeon.