Immediately after installing the Dark Souls: Nightfall demo, you will navigate the Demon Ruins and find yourself in the boss room of Demon Firesage. You'll notice a strangely empty, far end Source, and just as you're about to clear the room, the Fluffy Demon will block your way. Here's how to defeat the Blistering Demon in Dark Souls: Nightfall.
First, activate the Sprite mode by pressing Y if you haven't already. Blistering Demon and almost all encounters after that point are actually more difficult when using normal Dark Souls mechanics. The Blistering Demon attacks very quickly with a very short recovery time, so you'll need to learn how to attack and dodge, similar to the combat of Bloodborne or Sekiro.
Most of his attacks can be parried by dashing to his right, but if Bubble Demon raises his weapon to spear it to the ground, you must jump back. The hitbox on this attack is very unforgiving and automatically corrects to target you even if you get behind him. When he has about 75% health, he fires and gains several new attacks.
If the Blistering Demon raises his weapon and summons a glowing light, knock it back as far as possible – he's about to launch a massive magical AoE attack that will take 80% of your life. If it starts to spray fireballs you should go in and get a few free hits as it locks into that animation for a few seconds. When his lifespan drops to 30%, he'll fire more, gaining a faster attack speed and a five-hit combo that should be completely avoided – there's a powerful roar to be said for that.
If you take a hit and want to heal, you can trap the Bubbly Demon at the large roots on either side of the arena. But not only the biggest roots are broken, so make sure it's stuck in a running animation before turning off the Sprite mod to heal. Follow these tips and fine-tune your throws; You must neutralize the Bubbly Demon in a short time.