Finally Got It Yesterday!!!! It was the third time I try to unlock this achivement.
Now, I have the solution:
*Create a New Game alone no coop., never chapter select, just a New Game from the begining. If you stop playing, select CONTINUE the next time you play again.
* Play it on Casual, and go fast, I spent 5 hours.
* When Marcus gives you the customized lancer, NEVER, i said NEVER drop it. I use that lancer in all over the game until I unlocked the achivement. No Gnasher. Always with the lancer on your hands. Never use another weapon except in hordes , I used Troikas. Trust me, it´s easy if you play on casual with the active realoads.
* Take the ammo from the blue ammo boxes, never take the ammo from the fallen enemys, that lancers are not customized and maybe that could be a mistake
*Don´t reload Chapters!
*Don´t take a customized lancer from your friends.
* Just kill with your customized lancer and GO ON!!!!