This requires you to get to x10 on the animal meter in the upper left, which grows as you connect with combos. However, it slows down a lot around x7-x8, at which point you need to start varying up your combos. Note that there are 8 different combos in the game (check the Book of Knowledge); just rotate through them until you hit x10. (ie, do YXX, then XYY, then down+YXX, then down+XYY, then YYX, then XXY, then down+YYX, then down+XXY.) Keep up the rotation and you'll hit x10 without too much trouble.