I always wait for the zombie to have swung at me first (side step it), and then swing my axe while it stands there for a few seconds thinking about it. Sometimes the swing is off and you miss when you would think it was bang on. Other times you can seemingly kill a zombie as it runs past you and your axe went nowhere near it ^^ Try attacking in first person mode if you're not already. It's a must if you're punching or using a screwdriver as you can be much more accurate with your hits. Now the crouching ones on the otherhand....