Hentai vs. Evil (JP)

Zombies look like this:



Zombies are the single most common enemy in the game so unless you go out of your way to avoid them, there's no way you won't get this trophy naturally. The fastest way to kill a Zombie or any other enemy, is to shoot them in the head. Once you've killed your 250th Zombie, the trophy will pop. Note: I don't know if the Skeletons in the Pirate Bay level count towards this trophy or not.

See Box Collector III for a farming method should you still be missing this after finishing all 3 levels.