Om Nom is the green monster that you must get the candy into to complete the level. When the candy gets close to his mouth, he will open it. You need to get the candy close to his mouth ten times in row in one play of a single level for this achievement to unlock. You cannot let the candy fall-in to complete the level. This can be unlocked on many levels, but the easiest is on Level 1-7. From the start, quickly cut all the ropes, except the one on the bottom right (with the star next to it). The candy will fall and swing back-and-forth above the Om Nom causing him to open his mouth over and over. Just wait for the achievement to unlock.