Yeah, I also think that Toshiro would have a chance, but realistically speaking, what has he to offer? Ice and a Sword. And none of that will harm Percy, I believe. Keep in mind, there is not a single sustained wound under the Curse of Achilles in the Riordanverse, besides Kronos, whose full divine power and essence was resting inside a mortal, touching the most powerful weapon in the entire Verse, a weapon made to kill Primordials.
That's the only wound with Curse of Achilles. And keep in mind, under the Curse of Achilles Percy trashed Hades easily. Curse of Achilles is far more than just a durability increase, It increases every single aspect of a being to it's possible maximum, including powers as it seems. Considering that Percy was easily keeping up with People ranging from 6c to 5c+ throughout the series WITHOUT the Curse, I just do not see how Hitsugaya could possibly take down Percy. In Melee, Percy should be stronger.
Besides, it stands to doubt how effective Ice even is against Percy. He is noted to have "Ice Manipulation" and considering Water cannot harm him, I do not know if being frozen will even do anything.
I've added a list of the only things seemingly able to kill someone under the Curse of Achilles. This was copied from the Percy Jackson FANDOM page on the curse of Achilles, for conveniece sake. I however have taken out the "useless" onces that will only work for the Deity's in the Riordan-Verse
- Dehydration
- Asphyxiation (e.g. drowning, choking)
- Diseases (heart attack, strokes, cancer,etc.)
- Electrocution (claimed by the Rivergods, unproven)
- Burning (Kronos burned his hand on the molten Scythe of Gea/Gaia)
- Starvation
- Poison
- Exhaustion
Since Percy is Poseidon's Son it is quite doubtable he will die by Dehidration and he can't drown. He will not die by Disease in-fight either. Hitsugaya has no way to electrocute him. Neither can he burn him with Priomordial killing Metal molten by the Flame of Olympus either. He also has no poison.
So the only points on the list are Choking, Exhaustion and Starvation.
I'm doubting if any of that will happen. Percy is extremely able in Melee-Combat, so I doubt Hitsugaya will choke him to Death. That, also, would be out of Character for him. He either uses his ice or his sword.
While Percy can die of Exhaustion, he is extremely Tenacious und his Stamina is rechargable in water -or, if you want to look at it from another side, in water, he has nigh-infinite stamina. And considering Percy tends to run around with Ambrosia and Nectar, he probably will not die from either starvation OR exhaustion.
If someone had an idea how Hitsugaya could do anything against all this, feel free to share.
EDIT: Tried to fix Spelling Errors.