Hmm..... you need to be careful with him. It's all about attacking at the right time. You have to hit him the exact moment he's vulnerable and that's hard to do because:
1. You've been hit making you unable to attack when you need to (he's almost always on ground, and since you're usually knocked into the air, not even retaliating slash will help).
or
2. Doing so will GET you hit. His water tends to be in the way.
When he kinda "skips" (or whatever the heck it's called) to you while playing his sitar, that's when you need to be ready to use the reaction command. Enough combos on him will make him immediately retaliate, which hurts. You mostly need to dodge the water. If he sorta makes the walls of water "close in" on you, you need to wait until he's done until you can attack. I sudge ted jumping over him and arial dodge is neccessary and hit him. By luck he won't hit you, or it'll be the end of his attack, making him wide open. Dodge and attack. Pretty much it.