Rumbling Rose? What is special about that one?
Rumbling Rose has a weapon ability called "Finishing Plus" that allows you to unleash successive finishers in a combo. There's no natural way to get it without using the keyblade.
This is fairly unimpressive when you use it without any form due to the fact that Sora normally has only two ground finishers (Guard Break and Explosion) and one air finisher (Aerial Finish). However, if you use Rumbling Rose with Final Form, it tends to destroy a lot of stuff.
Valor Form does have some merits with Rumbling Rose with two ground finishers, two air finishers, a distance closing attack, the ability to use a finisher during a combo, and an attack after being knocked down, but the Combo Pluses will make it harder to get to the finishers in the first place.
Final Form, however, does not have to deal with that. In fact, if you also manage to have Fenrir in hand and a Negative Combo equipped, your second hit will probably be the finisher. There's joy in spamming Final Arcana to ram spinning keyblades into the enemy, Final Arts to launch them in the air with several dashes, and then Final Strike to hit them with dual flaring keyblades. You can also fire a bunch of Thunder if you don't have Berserk Charge set for even more chaos.
