Tekkai no Senshi

Tekkai no Senshi (鉄界の戦士) or Ashidaka: The Iron Hero is a Japanese manga series written and illustrated by Ryou Sumiyoshi.

Plot
In a world where everyone bears metal arms, a demon bearing a hundred pairs of metal arms threatens total destruction on the world and everyone who lives within it. But even in the face of total hopelessness, a group of warriors is ready to tackle this desperate challenge! A boy named Ashidaka, with two pairs of metal arms of his own, gathers his companions for the sacred war to come...