Hint: Level 5-5:
When you start, go back to the edge of the screen against the wall (under the key) and hang with rope. Once both peach-like enemies have approached and stopped above you, swing with rope all the way to the end of the white acid. Jump up on the small ledge and then fall back down again. Then, hop onto the ledge to your right and run all the way to the wall, pick up the key, and run back. To keep from being hit by the little balls that they throw at you, use rope to grab onto the pipe and reel up. Then, unlock the lock, but be careful of acid pouring out of the pipes.