Kill 100 enemies while in the power mode of Overkill.
Put your AI partner to passive mode and keep firing at the enemies. You need to kill at least 1 to max the aggro meter. After maxing it, continue firing to let a darker meter fill inside the red bar. Hold it for 2 seconds once it's filled and press to enter the Power mode of Overkill. All enemies will fire at you during this time but you do double damage. Kill 100 enemies to get this.