Yeah, this is not the same as Dexter's, so sneak attacks won't exactly work here, which is far from the end-all-be-all in this fight, but it's still somewhat noteworthy.
So if we're going to list all of the advantages each side has:
Kamen Rider Black Sun:
- Far higher LS
- Can detach limbs and use them as a sword
- Enhanced Senses/Extrasensory Perception makes sneak attacks useless
- Damage Reduction can somewhat reduce the damage he takes
- Regeneration helps him recover any damage taken to an extent
- Can increase the amount of damage he gives out
- Has weaponry that causes piercing damage
- Very high stamina
A-Town Down Under:
- Significantly higher AP
- Much more skilled
- Their finishers do significantly more damage than their normal moves
- Can overwhelm the opponent with attacks considering it's a 2v1
- Can overtax the regeneration by causing enough damage Kotaro can't regenerate from (or knock him out)
- Have ways to avoid getting choked out or escaping Kotaro's grasp
So based off this, Kotaro technically has more things going for him. He's far more versatile in terms of hax/abilities, and ironically enough, despite this being a 2v1, he can overwhelm A-Town Down Under with said versatility. But, some of the advantages listed can only go so far him (Damage Reduction, Regeneration, Statistics Amplification). Plus, Kotaro has a significant physical disadvantage by about 3-4x. Whenever A-Town Down Under land an attack, it's going to hurt. Even his biggest advantages such as LS can be avoided to an extent. However, I don't think Kotaro's LS will be the hardest task for A-Town Down Under to overcome. I honestly think putting Kotaro down for good will be the hardest task. His stamina is quite crazy, and Damage Reduction and Regeneration, while not the end-all-be-all, only makes it harder for A-Town Down Under.
A-Town Down Under has the overall physical advantage, having a 3-4x AP advantage in AP, SS, and Dura, so when they land a hit on Kotaro, it's going to hurt. It makes it a lot worse too considering it's a 2v1. Add their finishers, which do significantly more damage than their normal moves, and Kotaro's in trouble. Them being much more skilled also increases the chances of them hitting their moves, as well as being somewhat harder to tag, especially Waller, considering he's very swift and can casually dodge strikes and counterpunch thanks to his amateur boxing background. However, while they have many ways to avoid danger, Class M LS is still Class M LS, and that's going to be a bitch to handle. Not too mention having to actively avoid getting their skin cut via piercing damage. Putting down Kotaro for good is also going to be a very tough task. This match can go either way.
This is my unbiased/objective opinion on this match.