You can get this once you have the six permanent members in your party. This requires fighting 100 battles controlling each of them. Dezel and Zaveid register on the same counter.
You can get this in 200 battles by setting the rest of the party to manual in the artes screen, and controlling only one of them yourself. Once you hit 100 battles, switch the two seraphim out with the other two. In my experience, the game will suddenly reduce the HP of the non-controlled characters after a bunch of battles. I'm not sure if maybe this is some kind of way to prevent cheating? In any case, just feed your party snacks every ten battles or so.