It's funny, After beating the game multiple times, I don't even think the last boss is the toughest in the game.
I think Demon of Hatred is worse and I think you can make a compelling argument that Owl at Hirata estate is worse because he's dumb fast and it's tighter quarters.
The one thing about Sword Saint is that at least the arena is huge, so you can just run away and heal if need be. Plus, I feel if you are able to accurately reverse lightning, you should be able to score some easy damage on 3rd phase. Don't get me wrong, I'm not saying its even remotely easy, because it's still tough as nails, but I think the two mentioned above are way worse.