At the end of level 7 (Zombie Invasion) you will fly on a bat for the first time. This is a little bit harder than getting the achievement "Meep! Meep! Meep!" but not by much. Ignore the majority of the Sheriff Stars because they could get you hurt if you go for them. Kill flying enemies as soon as they are visible because they give you items that replenish your health. This shouldn't really be a problem if you focus on what's in front of you. If you are having a hard time getting this, replay the level after you have finished the game and lower the difficulty.