Here's my quick guide video:
This is a quick guide to show where to shoot from and where to aim.
Easy to do solo. Make sure to pack-a-punch the ray gun mark II - V (the lightning version). Herd as many zombies as possible, funnel them into a tight spot like where i shoot or through a door. Just hold trigger down towards lower body until trophy pops or all die. If all die, rinse and repeat. If playing on casual (recommended) it'll likely occur after Round 15 due to amount of zombies and their health so don't be discouraged if you can't get it beforehand.
Don't know how to build the correct ray gun? A great quick guide for that can be found here: