In Gears of War 3 for the Xbox 360, you can unlock the Instagib Melee mutator, which will make your melee hits destroy nearly every enemy with one shot, turning them into a pile of giblets, but cutting your experience gained by 25%.
In my opinion, this is one of the best mutators to unlock for Horde, because it allows you to kill nearly any enemy, even most bosses, with a single punch or Retro Lancer charge. Personally, I love killing Berzerkers with it, and not wasting a single shot on them. It doesn’t work on Brumaks, but they’re honestly not that hard to kill anyway.
To unlock the Instagib Melee mutator, you need to get 200 kills with the Wretch in Beast mode. This is probably one of the easier mutators to unlock, and doesn’t take a whole lot of time.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.
