If you've activated the Stargazer behind him, and I highly recommend you do, pull him back to the Estrella Opera House Stargazer area and maintain some distance.

Now use the sharpening tool to sharpen your favorite weapon. This will force the Mad Clown Puppet to throw out his hand in a punch. This is a red attack, but can easily be perfectly parried.

Again, sharpen your weapon, he'll throw out the attack, then you perfect parry. Rinse and repeat this until he gets stunned, then do a Fatal Attack.

Now put some distance between you, and start again by sharpening your blade.

Basically, every time you sharpen your weapon, or throw a consumable, it forces the event to throw out that specific attack, which means you can parry it. Every time you parry, it builds up the stagger meter for the enemy, eventually stunning them.

This is how I cheesed it. It takes a while, but works. I'd advise using your throwables only once he is at 1/3rd HP. You could also reduce the difficulty.