UPDATE watch the video it shows a slightly faster method the below method is find and you might want to switch it up a bit to keep from getting bored.
By the time I finished this took me 15+ hours but I was testing different methods and I only started with 3,000 kills. Leave comments with how long this method takes you and I will update my guide.
The best place I have found to boost for this is 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. You can go to chapter select and pick the Fallen Brother checkpoint.
Play on Easy.
The Traitor Guardsmen are very weak and only take two shots from your plasma pistol to kill. If you die or any of the psykers are killed just restart at the checkpoint and all of your kills will still count but I would be sure you got an autosave before you quit or turn off your xbox just to be safe.
This is also a good place to finish up some of your kill so many enemies using a certain weapon or ability achievements.
My advice is to start with the Autocannon which can be found right behind the first metal barricade to your right when you spawn leaned against a green box.You can kill multiple enemies with one shot from the Autocannon so aim for groups.This will also work towards your Firepower achievement for exotic weapons.
If you use up all the ammo in your Autocannon either restart checkpoint or switch to the storm bolter and you can find ammo for it right behind were you spawn. If you manage to run out of ammo switch to the bolter then plasma pistol. If your using the plasma pistol every time you shoot tap right bumper as well this will cool off your plasma pistol and shoot at the same time it will never overheat. These can count toward your 250 headshot achievement and 500 ranged kills achievement (I am not sure if the Autocannon counts toward these).
Tips for Combat
When your using the Autocannon fire in single shots instead of auto to conserve ammo you only need 1 shot to kill.
Blow up all the red barrels to clear away some of the boxes so you can get better shots.
You can shoot enemies before they are fully out of the warps just aim at the light and your reticle will turn yellow when you can get a kill
When your not behind cover constantly strafe left and right to avoid psyker shots.
You can stay right where you spawned and strafe back and forth or you can use the boxes and debris for cover here are some good spots
1. Behind the barricade you get the Autocannon from
2. To the right when you spawn there is a raised platform with a green box to hide behind
3. To the left after you spawn at the bottom of the ramp get right against the wall and metal box that has a psyker on it
4. Go to the furthest warp and hide behind the green metal box with a psyker on it to the left looking directly at the furthest warp you can just camp here and kill just the enemies coming out of that warp over and over or you can face the other way and you can get a good view of the warp to your right (left of your spawn) the enemies that spawn behind your will just run right past you and you can hide behind the green box shoot the guys coming out of the warp to your right then pop out from behind the box and shoot all the guys to the left just watch out for grenades