According to the search results, the weapon needed to defeat Colter is not consistent. However, the most common answer is to use the Thirst Zapper to weaken Colters shield, then switch to a high-powered weapon such as a minigun, mininuke, modified Alien Blaster, or plasma pistol to fire quickly at his head while his shield is down. It is also recommended to take a Fusion Core before entering the arena to weaken Colter significantly.