This can be very easy to miss on your first time through the game, but it is in fact possible to get this the first time you fight the Black Cerberus boss. You will have to fight the boss normally until he runs away and summons a mech, then hit the legs on the mech until the orange bar under its health bar fills completely. It will then hop away to try and blast you with vollys of cannon fire. What you will need to do is get the mech to blast the 2 giant white fuel tanks on the edges of the area with a cannon shot. Once the fuel tanks are down, destroy the mech and fight the Black Cerberus boss normally BUT you will need to perform an excecution on his arm to be the killing blow, which I'd recommend going after HIS right arm (your left). Failing to excecute his arm will still reward you with the head, body, and leg armor if you successfully destroyed the 2 fuel tanks AND kill the boss on the same try, but you will miss out on the arm armor, meaning you will have to wait until NG+ to try again. DO NOT KILL THE BOSS IF YOU CANNOT EXCECUTE HIS ARM IF YOU WANT THE FULL SET OF ARMOR. I hope this helps, and good luck.