Advice for Smelter Demon Boss? getting my ass whooped! lol
Haha, I told you.
Assuming you're using a melee-only build: Try to get in as many hits as early as you can, since he appears to buff himself after a certain period of time, rather than after receiving a certain amount of damage. Do not try to blocks his attacks after he buffs his sword (unless by some chance you happen to have a 100% fire shield at this time, which I did not). He will tear right through it. Instead, lock onto him and doge roll to his left (your right) as he swings. You'll begin to notice his patterns. When he's open (and in buffed form) do
not be greedy. With a slow weapon, one maybe two strikes will be all you do before backing up a bit to repeat the process.
The hardest part is that you will be receiving passive fire damage for simply standing near the demon after he infuses/buffs himself, and he's not prone to giving you a lot of openings to heal. If you have lightstones to spare, feel free to use them to help negate the fire DoT. Otherwise, if you're low on health, back up and patiently dodge his attacks, staying out of his passive fire range. If you see an opening where you would normally strike, don't. Don't heal, either. He will smack you right out of it. Instead, wait until he does his giant leaping attack, and stay within mid-range. This will bait him to do the follow-up (where he cranks the sword and makes a giant fire cloud that covers 70% of the arena). Walk out of that once you feel you've baited him by staying close enough (mid range). This attack has a huge recovery for him, and is the safest time for you to heal with your Estus Flask.
Your goal is to get through the fight without being struck, and reserving your health pool for the mandatory fire damage you'll receive simply by standing near him. I did this fight with my Bastard Sword and a Great Shield, btw.