I used D4RK I FuRy load point and followed these instructions: (btw thank you, your guide is sound to a point, no disrespect inteneded here)
Load chapter 5, checkpoint The Fallen. Make sure you have some crystal ammo and kill the blue tentacles. This will make it easier to defeat the rest of the enemies. They drain there power from the blue tentacles and will all be flashing grey now.
After killing them move on till you have a mini cutscene where enemies come out of the ground. This guys have the Krieger Guns. Go up the slope and turn left till you see another blue tentacle thing and kill it. Meanwhile DON'T kill the other enemies. Now they will all be flashing grey again.
Here is his solution that DID NOT work for me:
Search the one with the krieger gun and tap RT (don't hold RT). Quickly after that start tapping B. If done correct, you will smash him to the floor and pull of his weapon. Start hitting the other enemies with X while holding the Krieger Gun.
If the above paragraph doesn't work for you then this is what I saw and eventually did:
Go up the hill and shoot the first 3 Krieger gun zombies with the crystal ammo.. go to the blue tenticles (you should pass 1 remaining Krieger gun Zombie.) after you shoot the tenticles the zombies will in fact be grey and preped for the one hit kill. Now find the last Krieger gun zombie and grab him with RT.. and HOLD the RT button down.. he will be OUTLINED in RED while holding the RT tap the B button. this will make you take his gun.. now smash 4 regular zombies with the X button.. then let the remaining zombies kill you.. rinse and repeat till the achievement pops. (it shouldn't take too long)
If you do not kill the first 3 Krieger gun zombies they will make your life miserable by shooting you from very far away, they will also take out most if not all the regular zombies for you, so you won't be able to kill any yourself.. also if there are more than 1 Krieger gun zombies in the area and if you grab one with RT and hold it, it may be outlined in YELLOW and then it will break your grasp you won't ever be able to grab them again without using your Charge (Y) punch, which if you hold too long will kill them in one hit anyway and even if you do manage to stun them and grab them if it was outlined in YELLOW it probably still will be and will break your grasp indefinatly. That is why you should just kill all but one of the Krieger gun zombies.
*I wrote this just right after I got the achievement.