If you have not played the game yet, this achievement can sound a little daunting. It is, however, very easy to obtain.
Basically, completely ignore this achievement. There is a very high chance that you will get it naturally while getting the last of the other achievements. If you however have managed to get every achievement and still don't have your 10,000 kills, then simply head over to Survival and load up the first temple on easy. Let it build up a little bit so they start spawning more, then just sit and defend until the achievement pops! Will not take very long if you have finished the Campaign and every other achievement. It will unlock the instant you kill your 10,000th worker.