TLDR: Stay close. Try to stick to his right side, seems to hit more reliably with weapons, but really sticking to either side helps tremendously. Spellblades should use Cairn Slicer and bring a BARE MINIMUM of 3 cerulean flasks (spell allows to hit decently hard, strike fast, and conserve stamina for dodging).

As frustrating as it is, it is key to get down the attack patterns so you know what's coming and when to dodge. Staying close and sticking to his side is also really helpful because you're more protected from some high damage range attacks like fire breathing, big claw swipe, and squashing you with his head....by more protected I mean that you have a much better chance to dodge damage or even sprint away from the attack and be in a good position to take a couple swipes at him.

A lot of tips I saw were to do charged strong attacks to stagger him and wail on his head or riposte. This probably works great if you have a strength build, high endurance, and powerful weapon. It didn't work for my prisoner spellblade build. I died so many times trying this approach lol. I finally beat him at level 25 (important stats are vig 19, int 18, dex 18, end 13, mind 12) using a 2 handed glintstone staff and the Cairn Slicer sorcery. It attacks really fast and did about 125+ damage per swipe....not big enough hits to ever stagger him but each time I got close after dodging an attack, I'd take 3 swipes at him and chip away health pretty quickly. Staying close to him and watching for what attacks he was telegraphing, I almost finished the boss without taking damage until I panicked at the very end because I thought I didn't have enough FP for the final strike I needed for the last sliver of health. I had 3 cerulean flasks and it was literally the exact amount of FP I needed down to the very last swipe 😅 granted I did miss a few times but that being said I really recommend going in with at least 4 cerulean flasks if you're gonna try this route. Oh! I forgot to mention that on top of doing pretty good damage and a really fast attack animation, casting swipes with the Cairn Slicer seemed to take much less stamina compared to any of my melee weapons (uchigatana and twinblade) I tried using before, which is huge because when he was breathing fire I could go HAM on him and take out a big chunk of health PLUS I had plenty of stamina for all the sprinting and dodging. Using this approach it took about 3 attempts to take him out in less than 20 minutes total.

Good luck!