Throw a frag grenade in a group of three or more zombies. It will instantly kill them and you will get the "Zombie Roundup" achievement. If you do not get it from natural gameplay on your first playthrough, it can quickly be done at the start of the "B" scenario (2nd run) for both Leon and Claire. In the first room you enter is a grenade, only a few seconds after starting the "B" scenario story.