Dead Effect 2

On the second mission about halfway through, after getting the assault rifle there will be a special zombie wearing a spacesuit and throwing up. He will use the ultrasound wave that you need to avoid. You can take damage from his vomit and will not void this.

In the room the is massive glass and metal cylinders. You basically just want to keep looping that while taking popshots at him. He is very slow so you can run wait for him to come round the corner, fire, then move again and repeat process until he dies.

Doing this never got hit once.