I don't remember which one is 18, but I hate hate HATE the one with the timed jump-through-a-portal-to-hit-switch thing. And I don't think there's an easy way to skip only the one chamber. Beating it the first time was a pretty satisfying accomplishment, but it kinda ruins replays for me :[

I'm REALLY glad they chose to remove the harsh time-based puzzles in the second game.