Here's my detailed take.
The two of them start without seeing the other. Much of the fight's outcome lies in who sees the other first. If Hunter sees her first, he shoots and hits. If Suisei sees him first, she charges in no time and smacks him.
It's in the scenario if they see each other at the same time, where the result lies.
- Hunter's shotgun is a pump action shotgun, which requires a slide movement between each shot. If the first bullet hits Suisei, it's over. If it misses, there's trouble, because Suisei's fast movement would allow her to reach Hunter before he can slide again. That's without taking into account aiming, which would take a couple extra deciseconds. Suisei's options are:
- Suisei charges. In this scenario, Hunter has a better chance to hit, because of the closing distance. However, while Suisei is violent, she's not reckless. Running towards a gunner isn't a smart move.
- Suisei shrinks on the spot. This scenario guarantees a miss of the first shot, but then things are up in the air. Suisei would likely have to turn back to normal size to reach max speed. The positive for Suisei is that she gains a free couple deciseconds + likely a couple extra ones due to Hunter's surprise. The negative for Suisei is that she's revealed her shrinking, so Hunter now knows she can do that, and she likely can't reach max speed while shrunk. If Suisei decides to attack and fails to capitalize on this precious time gained, Hunter is likely to recover on time, but if she does, then she has the upper hand. After shrinking, she can also retreat instead of attempting a charge, which would reset the encounter, but with the knowledge of the gun: cover is more valuable to Suisei than Hunter, so its existence is to her benefit.
- Suisei tries to dodge. Because of the environment, stepping back out of view/behind the cover she just came from would be the most likely dodging option. This scenario greatly reduces the chance of the shotgun shell hitting Suisei. Afterwards, it would turn into a game of carefully chasing each other amidst the warehouse's covers. Here, the shrinking hasn't been revealed, so Hunter still doesn't know, which is a big plus for her. Against a gunner, using this ability would be a smart move, to hide and approach stealthily.
That's mainly in the scenario if they see each other while at 30ft apart. While they start 30ft apart, nothing guarantees they will see each other at 30ft apart. If they happen to see each other at closer range within reach of Suisei's weapons, it's to Suisei's advantage. And if they happen to see each other at a further distance than 30ft, the same options I described apply.
Finally, while more minor, Suisei's LS can be useful, specifically due to the environment. The option to toss containers and large boxes is there.
Taking all that into account, and based on the circumstances of the fight, location, equipment, weapons, and options available to each, I'm leaning towards Suisei having a higher chance on average.
PS : Genuine question: where's the idea Suisei's more associated with chainsaws coming from? If we go purely by number of appearances per weapon Suisei has used in HoloGra, her halberd/axe has the most.