G (Stage 4) is Claire's final boss, a grotesque, multi-eyed monstrosity. His weak points are the numerous eyes that appear on his body. Use your Grenade Launcher (Acid Rounds are highly effective) and SMG to destroy these eyes. He will frequently charge and attempt to crush you. Keep your distance and use the environment to your advantage. After dealing enough damage, Sherry will drop a Rocket Launcher. Grab it and fire it at G (Stage 4) to end the fight. This fight requires constant movement and precise aiming.
