This one can catch you off guard. Although the enemy is pretty slow with its attacks, it can do massive damage to you depending on the attack. There are also pits of fire around the stage that limit your evasion tactics.
When you enter the fog gate, head directly towards the boss. Chances are, he will use his right hand and vertically smash his cleaver on the ground. Dodge this by rolling towards him to the left so that you end up beside or behind him.
Once you're behind him, lock onto him. Now you can either risk hitting him once, or just wait for him to attack you. When he begins preparation for his attack, roll back. Most of his hits can be dodged this way, except for one attack where he vertically smashes his cleaver directly in front of him. It has far range so be careful of that. If you see him charging that up, try moving back a little before rolling back.
When you roll back, quickly roll forward towards him again and hit him as you're rolling. I would say hit him twice, but you need to preserve as much stamina as you can due to the fact that you're rolling back, then forward, then hitting; You probably won't have enough stamina to hit him again and roll back unless your stamina level is high.
If you feel yourself getting cornered, try rolling around him until you're on the other side.
When he reaches 50%, he gains two new moves. One is a dark energy cleaver blast where he charges his cleaver with dark energy and blasts it your way. It's hard to dodge this. Try rolling to the side as much as you can or running behind him if you're close enough.
The other move he gains is a dark energy radial blast where charges up dark energy and it creates a AOE around him. Avoid this by rolling back twice.
This battle can be tricky, but if you manage to beat it, you'll earn 'Soul of the Rotten' and a whopping 47,000 souls