When the fight starts, use your Voidwalker on his mace and then on the ground. When he brings it around jump through and attack his soft spot. He will fling you off and fall down, so attack away. When he gets back up, he will summon smaller demons. As you fight them he will do a wide sweep attack. Don’t bother trying to put a portal on his mace because it will go away when he gets ready to do his attack. He may also do a smash attack, but you will see his hand raise, so you can dodge it. Just keep repeating this pattern and he will die.