His attacks don't just all nearly kill you, even with completely maxed out stats, but they inflict you with crippling status effects, as well only these are different these can't be countered by Void. In fact, if you enter the battle equipped with anything that reflects or absorbs any element, Demi-Fiend kills you in one hit.
Worse than this is his infuriatingly strong Gaea Rage attack which, in a game with a HP cap of 999, does 7,000 damage! Unlike any other optional boss out there, you can only win this battle with a little luck on your side. The way to survive this attack is to equip Null Sleep, hope one of Demi-Fiend's minions casts sleep on a character at the right moment, and that that character then doesn't receive damage from the mega Gaea Rage attack while asleep. Oh, and if that wasn't enough, there's one more thing. His minions not only constantly negate the effects you cast on him, but enjoy healing him to full health from time to time, too.
this sounds in no way like a fun thing to do
its an exercise in frustration. its not gaming, its a chore. ---