This is a story-related, unmissable mini-boss fight that occurs at just over the single player campaign's half way point. You encounter an enemy faction known as the "Burnouts" while fighting your way through a shopping mall. You'll come to short bridge area where a helicopter crashes with its blades still spinning at high speed. Eliminate the initial wave of enemies and a "hyper-mutated Burnout" will climb over the bridge. Shoot it repeatedly with the shotgun to force it back far enough until it gets diced by the helicopter's main rotor and the achievement will pop soon after the fight is over.