Great news on this one... It DOESN'T have to be utilized against Brutes (the big ape enemies). Rather I wound up doing this solo on a level entitled "Security Zone" in Firefight Mode. You pick up a plasma pistol as there will be plenty and charge it up with RT and hold for a few seconds until it makes a consistently charged sound, then when you get a wave of Jackals (the lizard guys who shoot with carbines or beam rifles and have a shield to protect them) run up to them and release the gun, it will disable his shield and then immediately hit "B", maybe even twice at times. Do this nine more times before all of your lives run out and you'll get this in no time. (Also note to strafe alongside the barricaded area where you spawn because you can only get 2-4 kills per plasma pistol depending on the ammo, so you'll have to find more to swap out with.