To unlock this you need to get 3 kills with the Gnasher without dying 50 times.
Unfortunately these do not stack so getting 9 kills without dying will not count as 3 of your 50 times. So you will need to get 3 kills, get killed and then start again.
It also doesn't need to be 3 consecutive kills, so you could Gnasher 2 enemies, lancer another and then switch to the Gnasher for the 3rd kill.
This can be done in co-op vs AI (as can the rest of the mastery achievements). I found it quite useful to pair this with the Enforcer in your starting loadout as you need 5 downs per match with the Enforcer for that achievement and then you can use the Gnasher to finish them off.
There's also a glitch for the Hammerburst and the Lancer that needs Gnasher kills so you could tie those achievements together. I would check those respective achievement pages for those glitches.
Glitch was patched.