This is easy, actually. This can be done in the monk's puzzle. Right at the beginning you will get a heavy feather. Normally, you won't need it anymore after you broke the bridge to get into the temple. But take it with you! Now at the fourth task, where you travel on these flying carpets up into the air, you will have to get all three characters right at the end onto the right side in an alcove. There you'll go up the rope and stand with all three characters (including one holding the feather!) on that bridge. Jump with one of the characters, the bridge will collapse, you'll land in the water and the achievement is yours!