Smelter Demon

It will start with the basic attacks that you can easily block or evade. For its vertical slash, keep your shield up. For its downward slash, evade to the left. A few seconds into the fight, the demon will charge itself up. Use this chance to land a few hits on it. It will also upgrade its damage and defense after charging up, making it more harder to kill. Keep circling around it with your shield up. Attack or heal only after it finishes its attack. Finally, it will charge up one more time, this time its flames more potent and can cause fire damage when hit. Make sure to pop a lifegem when you're on a defensive to keep your HP up. Having a Ring of Restoration will also help offset the gradual HP decrease due to the flames.

Be careful when the boss jumps and its sword gets stuck on the floor. It will release a very damaging AoE (area-of-effect) blast so keep your distance while maintaining your lock-on. If you find yourself struggling against this demon, you can always come back for it later once you got better gear or spells.