Won Sul doesn't have essentially any amped durability. It's the durability of a highly trained human in the SAO-verse. He is immortal but it's pretty much useless when he ends up just being a face.
Gala has likely supersonic reaction speed and wasn't it the case that trained people were able to see Won Sul's killing intent weapon? I mean, I don't see why Gala wouldn't be able to, being a freaking NINJA MASTER and all. Having a cursed sword, emitting Ki that's even visible to someone like MacPain who is a few leagues below Gala and so on. With its invisibilty it loses its greatest danger factor, imo, because if you can see the exact shape, it becomes dodgeable. And speedwise I would still say Gala is above Won Sul.
If he uses his seven bodies technique it could even become a babyshake. Because even Ney couldn't see the real Gala and her perception exceeds sound speed movement as well.
Won Sul does quite some damage when he hits you good but it doesn't exceed that of the normal Shin Majin Ken strike. It isn't on par with celestial flash and it's also not on par with Kei Un Ki NinKen, which does this:
Going by the usual height of a castle of fortress wall and the massive width it has, Kei Un Ki NinKen is already of building buster class.