nyx avatar is easier than what you will thought
get minato omnipotent orb (messiah, breed like 20 of them, you are bound to have one) if you don't that is okay, but you had to have either messiah or orpheus telos with strong everything
buy him around10 ARMAGEDDON's (Satan and Lucifer/Helel(fes))
kill first three form, and the next thing is you kept on spamming your arma
dead in like 15 minutes........
WARNING : don't take mitsuru, take say Akihiko, Aigis, Junpei/whoever
this apply to ALL the difficult fight of persona 3 and fes except for elizabeth/margaret fights, messiah+omni orb + arma = minato the emo jesus death machine
izanami is way more belching