Boxer (S01E11)

The No. 1 Division were spread apart because of the Terraformars. In the depths of a dark hole, the gong had rung on a battle of the Earth vs. Mars. On Mars's side, a weevil-based Terraformar and on Earth's side, was Onizuka Keiji! He had always lived a very simple life. In order to protect the island his mother loved, and to protect his precious new friends, Onizuka stands in the ring yet again. Thus begins the brutal and unrelenting battle of fists. (Crunchyroll)