Ryota the Sly primarily uses uses his poison attack, not much else, so get in close quarters. When he charges attack him as he's vulnerable, it's a perfect time to knock him out or disable his attack. Additionally you can pull or strike him into his own poison cloud which is also deadly to him. Not only that you can evade his attack altogether but it's not entirely easy. When he's charging it don't run the other way before he fires, it will actually make it harder to avoid. Run at him and if you can get behind him right before it fires it will give you time to keep running as the arrow chases you and eventually it will disolve on it's own.
