The hardest thing about this achievement is getting the shrink ray. After you get lucky enough, you need to shrink the following zombie types(you don't have to kill them):
Normal zombie: no explanation needed
Shrieker Zombie: wears some sort of tulban and flashes you (you can also get the Blinded By The Fright achievement if you kill it while flashed)
Napalm Zombie: Zombie with flames, you can't miss this one.
Zombie Monkey: They come when there's a power up to grab. Wait till they grab it and shrink them after. (This will also net you the Monkey See, Monkey do achievement) You can't kill the monkeys sitting at the spawn as it won't count.
They all need to be shrunk in one game and the shrink ray doesn't need to be PaP'd.