An easy way to achieve 30 headshots in one stage is on Stage 3 (O'Bannon Farm). You will be able to use the Chainsaw Blaster throughout the level so you shouldn't have that much of a problem getting 30 headshots. Hold , aim for their head, then press to shoot. You'll know you pulled off a headshot when the zombie's head pops off with flying pink dust. I recommend going into your options and switching your "Blaster Aim" to 'manual'. Doing this will allow you to aim more freely instead of always locking onto the zombie's chest.