Kakugo no Susume

A series of natural disasters has reduced the world to rubble, with the survivors doing whatever they must to survive in a world gone mad. But one young boy, Kakugo, gifted with amazing martial arts and a superpowerful suit of armor by his late father, has been charged with making the world (or at least his school) a safer place. But his brother has a matching set of skills and equipment, and he's on a mission to bring peace to the world… by wiping out humanity!