10000 Kills Achievement
Neutralize 10000 enemies throughout your career
This achievement may seem daunting at first glance, but the second level of the campaign has you in the gunner seat of a chopper, and all kills in this level count towards your total. The level takes four or five minutes to complete, and has around 130 enemies. It is quicker, however, to use the checkpoint saves to your advantage. When you see the save icon in the top right corner of the screen, the game is autosaving, so if you quit out of the level you can exit and resume from that point. Perhaps the best part to do this is after you kill the four boat enemies. You start in front of a settlement with 12 enemies, then go into a short cutscene (which you can fast forward by holding in the Enter key). Then you arrive at another settlement with 10 enemies, before moving round and killing another 22 enemies.
After killing the convoy of five or six vehicles (or leaving, depending on how fast you took out the enemies in the settlement you may have to wait), quit out and resume. This should take around 90 seconds including menus and loading. Rinse and repeat, and in around 5-6 hours you should have all of your kills!