The Demon Firesage is a fight that should be familiar to you since you fought the Asylum Demon and if you fought the Stray Demon, except he has a few new moves. The problem with this boss is that his melee attacks have a huge range, so you need to be careful with that. His most dangerous attacks involve his explosion, and if you've fought the Stray Demon, then you know that they have a good reach in front and a little reach behind him. He can raise his weapon up in the air and then slam it into the ground, causing an explosion. He will also just hit his weapon to the side of his body (kind of at shoulder level), causing an explosion. He can also flying up into the air a little and come down with a butt slam. If you're farther away from him, he will most likely either use the one explosion move where he hits to his side or jump back slightly, then jump forward with a vertical melee attack. The ideal strategy for this fight is to stick close to him and try to force his butt slam attack, which leaves him vulnerable to one or two hits from your own weapon. Be careful of his other explosion move where he slams his weapon into the ground, since he usually triggers it when you're close and you might misjudge the range (move far away, since you can usually see it coming). The arena might cause some problems because of all the roots, so you need to worry about that as well.