Bouken Ou Beet

It's the Century of Darkness where monsters known as Vandels rule the world, and the Vandel Busters are the only ones who can fight them. When a group of Busters are killed in battle, they pass on their magic Saiga weapons to Beet, making him the strongest Buster ever. But Beet won't just fight the Vandels--he wants to destroy them and finally end the Century of Darkness.

(Source: Viz Media)