Play an Coop vs AI on Casual (KotH works best) for this.
Let the enemies alive until the active reload cooldown is gone (small icon far right to your ammo), reload your Gnasher and kill them. Will be done in 30-60 minutes.
If your tracker isn't moving, restart the game or do a hard reset/boot of your Xbox, it worked then for me. Don't worry, kills are saved and the tracker will jump right to the correct amount of kills.