Very very simple achievement although very time consuming. You will easily get your 10,000 kills before most other achievements if you are looking to 100% this game.
If you are looking to get this out of the way early, play campaign missions and the defense parts are very good. Especially if you use explosive or heavy weapons on the zombies when they are making giant corpse piles to get up the walls.
A good mission for grinding this out since there are 2 giant horde waves is the second mission in the New York Episode. If you get auto-turrets or similar don't place them since their kills don't count towards yours. Stationary turrets on the other hand are very useful as they will do more dps than most normal weapons.