Head to Chapter 5-4. Travel through the first section to the metal door, which you should go through. In the second area, head past the save point. You will drop down a level, and there will be a ? block down below. On the block, Helper Marios can be collected. Then, jump onto the block and edge towards the nearby tube releasing Floro Sapiens. Switch to the Bommer (Pixi), drop him, and wait until the head detaches. This will eliminate the enemy for 600 experience, plus coins and dropped items. This seems to be a much more effective way of raising experience than jumping on shells.