Once he is done, there will be nothing but bones left.
So I looked through a few of Arzon's fights in preparation for this.
He seems to for the most part fight in melee range, albeit he can go to mid-range if he notices that it is advantages for himself. What this means is, that I don't think he will keep outside of Skull Waves range. It should be mentioned that I don't know how good Arzon's flight is, but he doesn't seem to make that effective use of it in character given that I haven't seen him use it in any of the fights I watched (Unless you count air manipulation enhanced jumps).
In melee he would quickly get annihilated. It's called a "Bone Sea" for a reason. If he approaches Skull Wave for close combat, Skull Wave would just surround him with his bones, cut off escape routes through the several meter high bone ocean and have them flow in on him like water. Arzon can block that temporary through an air shield, but has no effective way of retaliating or escaping the assault. So he would just "drone" in bones, in the end.
Even assuming he immediately decides to keep some range Skull Wave has good options. For one thing Skull Wave has good methods for approaching fighters. He can launch dozens of Skull axes as projectiles and where they land he can use the bones of the axes to shape skeletons. That way he can attack and by passing the hat on to a new formed skeleton has effectively switched locations.
To that comes that spacing such a large amount of bones is difficult to begin with. So there is a good chance the Skull Wave will be able to draw into close combat, where he can win in the way detailed above.
I should mention another strength of Skull Wave: The fact that everyone assumes that he is a living skeleton or, if the original one is destroyed, usually a living pile of bones. In truth Skull Wave is really just the gem in his hat, while the bones are material for him to manipulate. In Avatar terms this means that most people would fight the water, not the waterbender. That fact might be the most potent defense Skull Wave has.
Of course if all his bones are destroyed he would still loose, even if the opponent doesn't focus on the hat. However, that is not easy since he can just let structures be shattered and then piece them back together with his bone control and due to how many bones there are. When
Ileana threatened to fight Skull Wave, he predicted that the ground would run out of nutrients to grow new Mandragora before the Ileana's would be able to destroy all his bones, despite the fact that they would destroy each other a lot. We are talking about hundreds of Ileana's here, so that task is not easy for 7-B Characters.
This means that a war of attrition is something Skull Wave is very good at, which brings us to the final point:
This fight is of significance. It is one where Arzon decides to fight using his air powers on range. As is noted in Arzon's weaknesses expanding too much energy knocks him out. What the fight shows is that continued use, like necessary if he wishes to fight without his fists for longer periods, drains him very quickly. He would simply collapse from using his powers too much before he is able to reduce Bone Wave's bones by any relevant amount.
All in all I vote for Skull Wave (I know, very suprising)
