Just defeated him using the parry/reposte method with my fatal estoc+5. I have to say it’s quite a satisfying way of besting him. If you want to be slightly cheesy you could also get a back stab in just after he gets up.

CHAD KING

Was going for the storm ruler method and he fell down the drop down where the bow is at. Somehow he managed to drop to his death while trying to path his way back

Just got a PS5, so newbie to Demon's Souls. However I feel like I accidentally stubbled across an even easier cheese version of the poison cloud:
After you get him to 1/3 of his health and he gives you demonbrandt, leave the area/ reload it. The second time he will have more dialogue and then start aggro. After that he will just aggro if you get close enough. I found that is you stand in the center of the second square of tile in front of him and cast poison cloud, he won't move, react, aggro, or even heal past half health. after about 15 minutes of repeating Poison Cloud he died. Unbelievably convenient after the MANY different cheeses and tactics I attempted. Hope this helps someone.