This is unmisseable, it will be unlocked by defeating a Gunk Beast in your first encounter with it in Chapter 5: Pure Lightning. If you have the highest upgrade to the Pulse Cannon, you can stun the enemy and then absorb the gunk glowing spot in his back 3 times to kill him. If you can't stun him with your pulse cannon, the trick is to make the beast charge at a rock or wall, which will make him get stunned and then proceed with absorbing the gunk spot the same way.