I was pretty sure the floor 22 guardian did it, but yeah, looks like you're going after one of them now.
Burninator is easier, so stay away from floor 21 for now.
Burninator easier indeed. For laughs, I tried it out. Party wipe first attempt (Oh god Flowing Hellfire you are NOT my friend!).
Second attempt preparation included equipping Remi, 18F character, Reimu, and one other person with plenty of FIR affinity equips as well as spending the skill points I had (~300-400k) to boost the heck outta Remi's FIR affinity.
Everyone survived Flowing Hellfire this time (Barely). Buffs up as usual...oh god it still hurts I may have to grind for more skill points to put into MND/FIR affinity when I die.
...Or should I say, I might have added more characters to my party that could cause PAR with their skills instead. Turns out Burninator is quite vulnerable to PAR. 7F girl's 3rd skill pretty much meant I could keep it in near permanent lockdown. It did fail to hit with PAR once, but I managed to survive boss's next turn.
Remi + 15F optional pretty much killed the boss thanks to being able to keep PAR on it.
Got what I could from floor 25. Seems like I do have to take out that floor 21 Sigil Guardian if I want to get anywhere.
Great, I get to figure out how to not get crushed by 21F Sigil Guardian's Demon-Slashing Dance while I grind up for said boss.