An interesting matchup, to be sure. However, unfortunately, I'm going to have to give this to Cagnazzo for multiple reasons. (BTW, I'm assuming this is the High 6-A keys for both characters, since this seems to be the most even looking matchup between the two profiles since you never specified it here.)
First, his status moves. While the majority of Groudon's movesets revolve around offensive actions, Cagnazzo has access to a variety of moves that can easily hinder Groudon to a screeching halt, such as Hold, which will prevent Groudon from moving; Slow, which causes Groudon to move even more sluggishly than before, and it wasn't much of a speed demon to begin with; or Sleep, which puts Groudon into a deep slumber.
And then, there's Tsunami, which is considered Water Manipulation, which would deal much more damage to Groudon. And while Drought could probably cut this down, not only would it not get rid of it completely like Desolate Land would do, but Drought doesn't last forever, and it will go away after a while. (Unless I'm reading too much into game mechanics. Correct me if I'm wrong about that.) And while Groudon could technically vaporize Cagnazzo's Tsunami with Thunder, it is one of the lesser accuracy moves, and even then Groudon wouldn't refer to Thunder every single time, especially if it's raging as it tends to do. It would more likely slink back to it's other moves, such as Precipice Blades or Eruption.
So, in the end, I believe the Final Fantasy boss takes this with mid difficulty. Don't get me wrong, with all of Groudon's powerful attacks, it would definitely put up a fight. However, Tsunami is basically instant death to Groudon, and with the added status conditions that would get slapped on by Cagnazzo, the Continent Pokemon will eventually fall into the sea.
Winner: Cagnazzo