Kirk vs Gorn

The Original Series called "Arena." In this episode, Captain Kirk fights a Gorn, which is a large, reptilian alien. The fight takes place on a desert planet after the Gorn attack a Federation outpost. Kirk uses his cleverness to build a weapon from materials on the planet, eventually defeating the Gorn without killing him. The story shows Kirk's strength and intelligence, but also his mercy, as he chooses not to kill his opponent.