Old Demon King can be found behind the fog gate located just behind the giant wyrm in Smouldering Lake. This can be a challenging fight for a melee class, as he hits hard and has a lot of AoEs. If you are having too difficult a time, I suggest respecing into a Sorcerer or Bow build and then respecing back when done. Ranged classes will find this fight a breeze. The key to this fight is PATIENCE. You will only get 1 or 2 attacks if you are lucky in most of your openings. If you try for more you will be hit by an AoE 9 out of 10 times, so gambling it’s that 1 time just isn’t worth it. For this fight you will be dodging EVERYTHING. You don’t need a shield here, I just happen to be using it as I like the the moveset of my weapon better when one handing. The strategy here is baiting him into attacking you, roll through his attack, hit him once or twice and then roll away as fast as you can. If you stick to that for phase 1 you should have relatively no issues. In phase 2 he will ignite and have a flaming aura.
His moveset will change up somewhat and he will now have more AoEs than before as well as a meteor spell that gives you a great opening to hit him and also a great opportunity to get murdered if you don’t dodge in time. When he jams his hammer into the ground it will create a flame shockwave that goes inward from about 20 feet around him and then back out again. You’ll want to back away and wait for this to vanish before engaging him in melee combat again. Again the strategy is the same, roll through his attack hit him once or twice and then roll away as fast as you can. The tough part about phase two is that you have to pick your openings a bit more carefully than phase 1, but generally it is actually a bit easier as his meteor spell gives you about 5 seconds of time to hit him as much as you can.
A note about the end: he will stagger at about 5% health. If you don’t kill him fast enough he will explode and one shot you. This is why I was being cautious here, thinking this was about to happen. .
As found on Youtube