Let's see...
-If you take ANY null-skills, you insta-die.
-He's resistant to just about everything except Earth and Almighty. (Since Earth didn't exist in Nocturne, clever of them.
-His demons infinitely respawn.
-After three die, he summons either a Titania or Pixie who tries to put you to sleep. Afterward he casts Gaea Rage. If you have Null Sleep equipped, anyone who falls asleep survives. If no one falls asleep, that's it. So you have to end it as fast as possible. You're lucky to survive ONE of these instances, multiples is a miracle.
-The first team constantly buff/debuff him, as well as Girimehkala throwing out Mamudoons.
-When Demi-Fiend is on his last legs, he gets even stronger.
-He has an insane critical rate, even with Null Critical equipped(which is one of the few he won't kill you for taking).
-If his demons survive more than 30 turns, they suicide and full heal him.
There's a workaround called the Red Star strategy where you can stun-lock his demons into running out of MP, so they can't suicide and full heal him, and you can debuff him to your hearts content and he's stuck with two weak demons, but you still have to survive one Gaea Rage to pull it off.