View source for PSP:Lumirus / Power S

The S RANK Lumirus/Power unit is actually +220 attack, and it can be found (while playing multiplayer) in the S-RANK mission: "Claws and Bullets". It's not a creature drop item; as it can only be found in one of the boss/clear level crates after defeating the Alterazgohg.
 
side note: I was a either a gunmaster or fighmaster when I found it, and I was only playing 1P.