In the first shutter-blocked room, start by bombing the top wall. You'll find a key and a door that cannot be reached by any means. Go back to the shutter room and clear the enemies, but do not advance. Rather, push around on the blocks until one reveals a staircase. This will take you to the unreachable side of the key room. Go to the west and defeat the Gleeok to find the Magic Boomerang. Then return to the key room, press against the north wall to pass through, and defeat the Manhandla to return to the start of the dungeon.