G-Force

I unlocked this achievement at the checkpoint "FBI Computer Labs" where my objective was to get through the frozen FBI offices, and I got the achievement here by killing all but 3 enemies that I have come in contact with.....

If you don't earn the achievement by this point, and you've killed most the enemies you met, you're most likely close, so just keep killing and it is bound to unlock around this point.

Another easy way of racking up kills in this area, is to unfreeze the microwave that is stuck in the ice using the Fire extinguisher to burn it out, then just keep killing the little spider things that keep respawning, these creatures count toward your 1000 kills.

You should get this achievement if you kill every enemy you see in the game. I unlocked this during the RDV Escape mission only missing a few enemies. Save your game once you get here and reload the save once you complete the level if you don't get the achievement.