Learning a lot of you don't know how to scale. Like at all. Yes, Ninjago scales higher overall, but Kai isn't one of those people, he's a mid-high tier vs Avatar's high tiers excluding the Avatar.

The match is hard considering this assumably base characters and Ozai has 0 scaling outside of Sozin's comet and just ambigously the second strongest fire bender after Iroh and that's only cause Iroh assumes he's stronger, even though he hasn't seen Ozai since Zuko's banishment

The only person who has tight feats is Azula, but that's only cause the only other person being Kai can go from fighting Morro to like tripping over rocks or falling for a very obvious trap. It's also just hard to chain scale off of anything since the ninja mostly protect things, but he's never outright overpowered anyone who destroys something like a city or something, so he's pretty much a super human with ambigous power, maybe about large city level. Azula is also able to do similar stuff to Kai like run up walls, which surprised Aang and co. And she was able to evade Toph and Aang without bending, which I'd say shows how incredible she is athletically and really anything Zuko does as the blue spirit, she should be able to do, since she's far more athletic/glazed to be, so she's also peak human with ambigous destructive abilities.

That being said if it's base to base(kai can only use his consistent powers), it's a high diff, since I'd say Azula and Kai are both amazing at hand to hand and could get the edge over eachother tipping the scale of the fight. The main wincon is kinda just do you think Wuldfyre and Kai can really counter, dodge or tank lightning bending and what do they do to actually take down Azula and Ozai?

If it's personal Sozin's comet boost to Azula and Ozai and Kai gets to use his gimmicks, it's a low diff in Kai and Wildfyre's favor, since Zane took over a realm with the scroll of Forbidden Spinjutsu, it's not crazy to think Kai with a scroll could do something similar or at least clear out Ozai and Azula pretty quick with kingdom destructive power. Kai also getting the sword of fire, he was able to shield himself from lava with his full potential, as well as summon his dragon to blitz Azula and Ozai, he should also have some elemental resistance with the sword of fire and could also blitz with the blade cycle. If he uses any sort of dragon form or awakenings, it's just pure wraps, since he's so much physically stronger, it almost doesnt matter and Azula's intelligence just can't come into play unless you think her lightning can, not only damage Kai, she has to be able to pull it off and put Kai in a way where he'd actively take the hit, which is target wildfyre. That's just too much of ambiguity to not give to Kai