I believe I've found a great place to grind this out relatively quickly if you find yourself done with the rest of the game and still need a few (hundred) kills. Go to Chapter Select and choose The Construction Site on Easy.
As soon as the level starts, head right and jump onto the police car. From there jump to the right toward 2 zombies wearing police uniforms. During that jump, throw a grenade with LT, and throw another as soon as you land. If done correctly, both will land in a large group of zombies just past a pile of boxes. You can use the default equipped weapon to kill the 2 zombies near you while the grenades blow up, then immediately pause and hit "restart checkpoint" and repeat until you are bored or get the achievement!
If you haven't got them already, this is also where I finished off blowing up 10 cars (using the police car I mentioned jumping off of) for:
As an added bonus, this is where I managed to kill 10 zombies with a single grenade for:
Just wait a second and throw only one once all the zombies past the car are grouped up. In case you are confused, killing 10 zombies at once with a single grenade is what that achievement is actually looking for (its very poorly worded). Happy zombie slaughtering!