The NinjaG 10

To unlock this achievement you simply need to execute 50 zombies with stealth kills during a Breakdown level 5 or higher match. A stealth execution counts towards this achievement when you down a zombie and perform a finishing move by first holding down the B button to go into a crouching stance. Then you will need to sneak up behind a zombie without been seen and pressing LB + Y once you are close enough. When this is performed correctly you will grab a zombie by the head and drive them into the ground for the stealth kill. If you are near objects like a fence you will see a different animation which will always end with a zombie's head being smashed into something. Complete this special move 50 times and the achievement will be yours.