One Whole Ghost are a pain in the neck, aren't they?
A bit of luck is involved if you want to beat it at a low level - as yeah, not getting poisoned makes a HUGE difference (At that low of a level, poison is practically a death sentence). Your best bet is to keep Reimu using the Great Hakurei Border to try to keep your party's defenses up, and don't be afraid to swap out Marisa, Patchy, Chen, etc. as necessary. You have a paralyzer now - use it! You should also have gone and defrosted a new ally as well, who can act as healer (Just keep her defenses up too, but she can buff defenses as well).
It's the first honestly hard fight in the game if your luck is bad.