At a certain point in the story - during the ‘The Red Mist’ quest, which takes you into the Brentwood Sewers - your character will awaken their Fury ability. Fury allows you to enter a powerful rage state, enabling you to tear enemies apart with your bare hands. To activate Fury, you need to first fill the red skull gauge in the top left corner of the screen, by killing zombies and consuming energy drinks, then click the  and  thumbsticks simultaneously. Now, simply go apeshit and claw those undead to pieces. You’ll work through 50 zombies like a knife through butter in no time.