To unlock this boss fight, you must defeat Ponce de Leon and then obtain Ponce de Leon's Obol, which is a random item you might receive when praying at a shrine.
|One of these things.|
Next, you need to find the Tower boss room. Usually the doors to boss rooms will be closed after you defeat the boss, but once you give Charon the corresponding Obol, the room will reopen. If you enter, you'll be taken to this special remixed boss fight. Failing one of these fights will drop you back outside the boss room, so you can retry immediately, making it very easy to practice until you can get them all done.
The remixed bosses are the hardest challenges in the game by far. In each, you'll play as a special character, so you can't just grind out gold to get a better dude.
|A shot of about one quarter of Ponce de Freon's arena. It is as wide as Ponce de Leon's arena, but missing most of the platforms. There are, however, a few platforms along the top of the area.|
and he comes equipped with several runes:
6 Vault (a bunch of air jumps)
Here's a screenshot of his stat page, if you'd prefer:
|It feels like there's a reference in here somewhere...|
Ponce de Freon's moveset is identical to his brother's, though the fire trail he leaves takes longer to dissipate and the fireballs may be a bit closer together. As such, the best strategy is basically the same: try to lead him around the stage so that the first fireballs start disappearing by the time you return to that part of the map.
One boon in this particular fight is that you don't have to contend with anything else in the arena - the spike balls from the fight with Leon are absent here. That means you can comfortably focus on avoiding Freon's attacks without worrying about ramming into an off-screen enemy.
Another important note: you have to perform a series of air jumps to reach the floating platforms and circle the stage. When you get pinned against a wall, jump up to the ceiling and continue looping.
Because Sir Dovahkiin is a Barbarian King, you can use his Barbarian Shout to clear lingering fireballs. You only have enough MP to use this ability three times, so you need to be somewhat conservative with it, but it can get you out of tight spots when you really need it.
Unfortunately, there's not really much else to say about this particular fight. Here's the video of my successful fight against him to give you an idea of how it goes: