it was this level, specifically the trick with the red barrel in the beginning (you shoot the barrel instead of waiting for the Green Buttslam Move to blow it up, since you are high above the ground when barrel blasts you upwards you get just high enough to reach the lip of the waterfall, and then you can jump off the waterfall with the momentum boost water gives you in this game. this lets you keep the Yellow Gun to use for some other shortcut later)