Okay I struggled with Ryota the Sly as a lvl 13 warrior on hard until I figured out the secret.
Upgrade Brutal Advantage, Iron Discipline, your potion, and Heavy Armor all the way (if you can).
When Ryota the Sly charges his poison throw get close to him and block.
The poison cloud will cover both of you. You will take some damage from the inital hit, but you and Ryota will take damage from the cloud. As quickly as possible walk out of the cloud.
Use your normal attack on Ryota if he's standing in the cloud. If your Brutal Advantage is active Ryota will end up stunned in the cloud and take a lot of damage, while you are free from the clouds harm.
If Ryota is in front of the cloud use Slam to knock him back into it. If Brutal Advantage is active Ryota will be knocked back into the cloud and stunned again taking damage.
When you can heal with the potions (although the healing effect will be reduced. It might be helpful to put a point in Blood Mirror to dispel the reduced healing before healing with your potion.)
The Iron Discipline skill is helpful because you most likely will get at least on critical hit during the battle. Criticals restore health and focus with iron discipline.
This took me awhile to get it down, but when I did it wasn't too difficult to defeat Ryota the Sly.