Warhammer 40,000: Space Marine

You can get this in chapter 14 after Nemeroth impales Sidonus a bunch of psykers will open warp gates and an infinite number of Traitor Guardsmen will come through. Grab the Autocannon to the right after you spawn its leaning on a green box by some debris. Kill the psykers and four Chaoes Marines will spawn. Gun them down with the Autocannon but leave one Traitor Guardsmen and then restart checkpoint. I chose this spot because the Autocannon makes it easier and it is the best spot to boost for your 40,000 kills.

You can grind this one out in Chapter 12 - Dying of the Light.

From the beginning of the chapter, run through the level and you will come across a skirmish pitting Chaos against Ork (Chaos on the left and Ork on the right).

At this point, there are three types of Chaos forces: the fiery demons, the 1-hit-kill soldiers, and the Chaos Marines. You will know the Chaos Marine ... they look similar to you and it takes a few hits before you can execute/struggle with them. There should be 2 or 3 of these Chaos Marines in this first mixed skirmish. Kill them and reload the checkpoint.

However, if like me and you accidentally go overboard, you will soon come across a battle with only Chaos forces. They all portal in/spawn across from you ... run across the bridge, kill the Chaos Marine here (and it is a good idea to kill everything here at this end of the balcony), then head to the other end of the balcony where there are a few more puny soldiers and a Chaos Marine. Take him out and reload checkpoint until you get the achievement.