Shin Ishu Kakutou Taisen: Chijou Saikyou no Seibutsu wa Dare da

Hagane has attained the title of the strongest man in the world after becoming the champion of the ultimate mixed martial arts tournament. After winning the tournament, he was contacted by a mysterious man who questions Hagane's title. Taking up the challenge, Hagane follows the mysterious man to a far away place, a place where he will face a new type of battle. He must now face the strongest creatures of nature, ranging from the massive elephant to the king of beasts, the lion.

(Source: MangaHelpers)