Party's over. Patched as of now. Thanks, based TC.
There seems to be a glitch that counts Instantgibs with the Gnasher as 5%progress IF you already have 10 assists/kills in that match. Preferably KotH
Here's how it actually works. Credit goes to Unmet Player
This will only work if you ONLY have 10 kills/assists with the lancer in the match. That means that once you get your 10th kill/assist with the lancer in the match, any additional kills with the Gnasher during the match will each get you 5% toward this achievement (Dying does not affect the glitch).
If you get your 11th kill/assist with the lancer, this glitch will no longer work in the current match, and you will have to continue in a different match.
It is helpful to leave the achievement tracker on and to mark this achievement as a favorite so that you can tell when you get your 10th kill/assist. Once you see a jump in progress indicated by the tracker, you know that you hit your 10th kill/assist. I would recommend to take the killing/assisting slow near your 10th kill/assist because,sometimes, the tracker takes a few seconds to update.
This glitch also works with the Hammerburst. "