Can anyone help me out? 
 
Here´s my situation:
 
My party is Yukari, Mitsuru and Akihiko. All lvl 76-77. I fight Nyx and use Mediarama after every 2 turns he does, just because they hurt a lot. This works well until he starts charming. I tell everyone to heal/support and eventually one of them decides to use a dis-charm, so I live.
 
Then it starts breaking out the Power Charged God Hand. This easily insta-kills me. If I try and prevent it by lowering it´s attack or increasing my defense then I´m using up my turn not doing Mediarama, which kills me.
 
When I tell Akihiko to heal/support he´ll just cycle trough his buffs, Mitsuru and Yukari just use Diarahan. So I´m the only one with a working Mediarama.
 
I need to do something totally different, but I can´t because somebody needs to heal, and the stupid AI sure as hell isn´t going to. I would easily win this if I had control of all characters :(
 
(Speaking of stupid AI in P3, Mitsuru has just enough SP left for a final attack on an almost dead monster, and she uses Mind Charge)

Hmm, I don't know off the top of my head.  My crew of Yukari, Mitsuru and Aigis were at least 5 levels higher than you're group is now.  I used Aigis (82 in my game) a lot I think as a support character with Yukari, Mitsuru and my toon (level 92) as offense.  

Just try to get to level 80 if you can. At that point, you can fuse the best Personas in the game, two of which can do a fusion spell that automatically causes 9999 damage. If you don't have a save before the point of no return, I can only recommend you find persona that have either Tetrakarn or Makarakarn. I don't think Nyx can absorb spells or physical attacks, he can only block them depending on which arcana he is. Your level for the battle is more than adequate, but if you're playing on Expert then I can understand why you're having trouble.

" Just try to get to level 80 if you can. At that point, you can fuse the best Personas in the game, two of which can do a fusion spell that automatically causes 9999 damage.  "
 

@Sleepy_Insomniac

" Tetrakarn or Makarakarn. "

get a Saki Mitama with mediarama and divine grace. You can heal your entire party each turn for very little MP cost. Leave most of the attacking to your allies (rush button is your friend). When she uses the reflecting attack, set everyone to standby and use the opportunity to heal and recover MP.

I had Yukari as well... but I can't remember the other two members I had... I think it was Ken and Doggy-Dog. Whatever my main party was, we were all level 99 by the time i fought Nyx (legitimate means, people.) Not sure what too tell you, but here's a little horror story scenario to try and avoid. 
 
On my first try, after suffering through this fight longer than I care to admit, Nyx is finally on the brink of death (as in, the health bar is NOT VISIBLE). One more turn and my long journey would have come to a joyous end, and here comes Nyx's last turn. "Nice try Nyx, but nothing can save you now! HAHAHAHAHA *heroic pose*", or so I thought. You see, Nyx decides to throw out one last charm spell, just for the hell of it. Not a problem so far. It hits Yukari successfully. I still don't see a problem here. Yukari goes next, and casts a full heal. ON NYX. This is the single most soul-shattering thing thats ever happened to me in my gaming lifetime. I try to keep fighting, but at this point Nyx is back to full health using his full-power abilities from later stages in the battle, and soon all is lost. 
 
I came back the next day and beat it, but the exact same thing happened again, except this time Yukari's heal missed Nyx. Good thing too, I might have stopped playing altogether if that happened twice. So yeah, be aware that something like this is a major threat later on.

" @Ventilator:   Using armageddon cannot be called cheating,since players must work very hard to get those two personas(Helel and Satan).Also,that is the only way to defeat the hardest boss in P3 game. "

" @Catolf: "Hardest boss" does mean Elizabeth. Nyx can be defeated without Armageddon, and I would encourage people to try beating her without having to rely on it. "

" @Catolf:  I'm pretty sure using Armageddon is needed to kill her. And just going and using it right away isn't enough to beat her. "

Elizabeth has some unspecified yet strict conditions for this request aside from requiring the Protagonist to fight alone:

• No Persona with more than one Null/Repel/Absorb property is allowed. Only Resist skills can be used to cover any applicable weaknesses.
• No accessory that nulls any kind of Physical/Magic attack is allowed.
• It is forbidden to use a Persona whose singular Null/Absorb/Repel property counters her own currently summoned Persona.
• It is forbidden to use the Armageddon Fusion Spell at any point in time. (except when her HP is at around 9,999 or lower, to which she can no longer retaliate after being hit)

If any of these conditions are violated, Elizabeth will summon a Pixie to fully heal herself with Diarahan, and also use a modified version of Megidolaon which deals 9,999 damage, well beyond the maximum HP that the Protagonist can attain. One casting means instant death. If the Protagonist is auto-revived by an item, Persona skill or accessory, Elizabeth will just repeat the vicious attack to do him in.