I had a bit of trouble with this guy, so I decided to write a guide. Hope it helps.
Getting To Him
In the catacombs (area with all of the skeletons) you'll eventually come to a rope bridge that doesn't look too stable. Start running across the bridge and a bunch of skeletons will spawn behind you. Run to the other side and hit the bridge once with your weapon. The bridge will weaken and the skeletons will cause it to collapse. Wait a few seconds and you should then be able to climb down the broken piece of bridge.
You'll then face a mini-boss demon, who is fairly easy to dispatch. After the mini-boss you'll come to a big open area which resembles a fiery lake. There's a massive ballista on the right side of the area that will constantly fire 3 bolts at you.
Run from cover to cover to the left side of the area. A massive worm will emerge from the ground. Run past the worm and enter a small opening on the right side of the wall to arrive at a bonfire.
After resting at the bonfire, go back towards the worm and take a right when exiting the cave. You should see a fog gate at the top of some stairs!
The Fight
If you have a shield that can block a lot of fire damage it will help for this fight. Unless you have very heavy armour and a huge shield, do not rely on blocking the demon's attacks, you'll have to dodge instead.
Phase 1
I found it easiest to bait the demon's horizontal attack. Stay mid range directly in front of the boss and wait until he pulls his mace to his left side. Then, roll through the horizontal attack to the boss' left back leg, take a couple swings and roll away once, bring your shield up. Most of the time he will bring his mace above his head and smash the ground causing a shockwave if you successfully dodged through the horizontal swing. If you got far enough away it will cause no damage, if not, the shockwave is block-able.
- If the boss starts to swing again instead of the shockwave, get as far away as you can because he'll go into a 4-6 swing combo.
- If he starts breathing fire, sprint to his left side - he will always breath from right to left.
- If he breaths a burst of fire in front of him, either be far enough away or block the blast (taking some damage if you don't have a good fire shield).
- If he does any over head smashes, do not try and block these - you'll get crushed!
Phase 2
Phase 2 begins when he is near half health. The demon will lunge forward and glow a bit brighter. When this happens, get in as many hits as you can.
The boss will do the following in this phase...
- Fire Rings: He will pound the ground with his hammer and a fire ring will emanate from himself or towards him (depends on your distance from him). The ring will pulse out then in if the prior and in then out if the later. It is not possible to block the ring, but you can roll through it. I usually try and stay far away from him until this ability is over.
- Meteors: If he starts shaking his hammer in the air, this is your queue to get ready to attack like crazy. Right when he starts waving the hammer in the air, sprint towards him and try and use all your stamina to attack, leaving enough to role once afterwards. Role once, then run away. A bunch of meteors should land near you. Get ready because he may do this again right after. If you do not attack him during this ability, he will continue to summon meteors indefinitely.
The above abilities are usually only triggered when you are not in melee range.
- Melee: The boss will do the usually slams and swipes as well. He will not breath fire in this phase.
End
Once the boss is very low health, you'll see him drop to his knees. When this happens, run as far away as you can! He will explode which will probably one shot you if you're not far enough away. Once the explosion is over - run in for the killing blow!
Hope this helped. Like I said, he had given me the most trouble out of any of the bosses at the time. Let me know if I can improve this guide at all. :) Good luck!