Alright, so I think here's how the fight would go.
The fight starts with a distance of 5 meters between the fighters, both assume their fighting positions. By the time the fight starts, Ayanokouji starts his analysis on his opponent, being able to determine three things:
1. The opponent has a chainsaw which can give him permanent injuries.
2. The opponent has a range advantage of a many centimeters.
3. The opponent's posture doesn't give the deductions of her being a trained fighter.
Ayanokouji realizes that his opponent is not a trained fighter, determining all of it as an advantage to his situation.
Both Ayanokouji and Suisei charge at each other. Suisei tries to attack Ayanokouji really fast with her chainsaw, which she swings very fast at an unbelievable speed, Ayanokouji dodges it. Now, he deduces one more thing, which is the fact that Suisei has strength enough to swing a chainsaw very violently and fast.
Suisei continues to attack Ayanokouji, while he dodges her every attack (Yes, his dodging abilities are insane. He has never been touched for roughly 30 volumes except for letting himself get touched deliberately or when he is in a tight situation, which are not like the situation he is currently in. Also, the fighters who fail to touch him are in reality highly trained fighters with a superhuman potential.). In the process, Ayanokouji proceeds to get closer to Suisei. He attacks her abdomen muscles, giving her immense pain, and getting her to crouch down on her knees, and stunning her for a matter of a few seconds. Then, he proceeds to attack her wrist joints, around the region of her carpals, attack the correct points to affect muscles such as brachioradialis and flexor carpi radialis (the funny thing is that Ayanokouji canonically knows about these muscles, and he should know this easily considering how he had already passed the curriculum of university levels). Ayanokouji then proceeds to disarm her and takes her chainsaw. Now, Ayanokouji can also use her chainsaw if he wants, but it is against the rules (but in character, it is more likely that he would use the chainsaw to his own advantage). So, Ayanokouji destroys the chainsaw, he basically has enough LS and AP to actually damage the chainsaw to a good amount and render it unusable. Suisei has returned back to normal now, except that her dominant side wrist is harmed and also her muscles are also damaged.
Ayanokouji proceeds to shower her with attacks, spamming pressure points and giving her immense pain. While she doesn't have any much endurance feats listed on her profile, I do hope that her pain tolerance is actually superhuman level. She also tries to attack Ayanokouji in the process but fails. Ayanokouji exploits her pain tolerance levels and continues to give her pain, while repeatedly using his bloodlust and engaging those emotions to attack her (Ayanokouji's emotional engagement is high, so of course, though this might not get considered a cognitive task). Ayanokouji, in the end, proves to be fatal and wins, but it would actually take a minute or so for him, considering how pressure points can help in damaging higher AP opponents.
Another possibility would be Suisei turning smaller. The thing about this is that he wouldn't do it when she has the chainsaw for sure, because it wouldn't help her grip it, so she would most likely do it when her chainsaw is dropped. But as Zetsu said, Ayanokouji can crush her, and she wouldn't be able to attack him. The most she can do is run around which I don't think would trouble Ayanokouji in any way, if she is not attacking him.
