I didn't get mine till the battle with siax, and I just though the game screwed up.

Really, if you really need final form, go to a save point anywhere when you have a full drive bar, and just go into drive. Then just go back to the save point and leave the world, come back, the rinse and repeat.

But I really don't think you need it though, I was able to get through all major orginazation members without it.