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Head to the right. Leave the floating legionary alone, you need him in a minute. Lead him over to the end of this area and make him drop a projectile so as to break the blocks. Kill him. Head onwards and grab the red potion from the pot. Go into the cannon and drop a potion. Do the same with the next cannon. You'll end up in the clouds. Hit the right-hand pot for twelve random items, then hit the left-hand pot for the key (if you hit it first the other pot disappears). Head right and unlock the door.