Persona 3 FES (Low Level [64], Hard) - Nyx Avatar 

Rules for this challenge:
-Goal is to beat the game while minimizing EXP as much as humanly possible (without losing my sanity)
-Hard mode, True Ending, No Nyx Skip, No New Game Plus
-All other mechanics are fair game!
~The ban on stat gains from incense cards and arcade is now lifted after beating all the Tartarus guardians, though it hardly helped at all in this battle as I hardly even rely on my stat grinded personas. LOL.
Played at double speed to make it more watchable for you, but I won't blame you for speeding it up even more :)
Because of how damage scaling works in this game (Player's LV vs Enemy's LV plays a big factor in the damage formula), Nyx's Avatar's Almighty Attack and even Maragidyne as early as Magician phase deals too much damage to reasonably beat at Lv 56. Additionally, my party members are dealing pitiful damage even with boost/amp passives because of their pitiful mid-tier skills, and their healing is hardly helping, even with Yukari's Mediarama. Inevitably, I had to increase my level to a point where I can more reliably survive attacks and generally just get better skills for my party (specifically Mitsuru's Bufudyne and Ice Break).
Why Lv 64? It just happened to be the level I reached when I got everything I needed for this fight. Mainly, Odin's Heart Item, Draupnir (Divine Grace on user). This made Yukari's healing much more viable and dealing with Nyx's damage output more reasonably manageable. My initial strat was going to be dealing most of my damage through Break spells and repel damage, but then found out about the glitch that can remove Nyx Avatar's second turn. This glitch is done by pushing Nyx into next phase through repel damage _from non critical HP_. If Nyx Avatar is in critical health before repelling damage back, it will still have 2 turns. Realistically, needing to break Nyx's immunity and set up a barrier to repel damage back is too cumbersome with just the protagonist as Nyx's 2 turns will make Break spells wear off faster and Nyx can still onslaught you while you're trying to set up. It's easiest to set up this combo on Priestess where you can set Mitsuru to Heal/Support so she'll cast Ice Break while setting up Magic Mirrors on the protagonist's turn. So I guess that's another benefit to leveling my characters to their 60s.
The fight becomes much more manageable as you no longer have to worry about double Almighty Attacks or a Ma-dyne spell into an Almighty Attack. This fight can still be dicey at certain points though such as phys phases where he can cast Revolution and Power Charge. Both of which work with its strength-based Almighty Attack. Instant death spells are a chore to deal with and a Mind Charged Megidola in the last phase before Death (Hanged Man) can put an end to your run before you even get to the true challenge of this fight.
For full transparency, I did use a save state at the start of Death phase. However, this was after many unspliced attempts being ended by Night Queen. On my successful attempt, I got lucky and Nyx NEVER used Night Queen. This isn't an uncommon occurrence, as I've seen it happen on normal runs of the game as well and I've seen other people experience it. I don't know how it works, but from speculation it seems as though Nyx can go into two different modes when you get it down to its final sliver of health. It'll either spam Night Queen and Almighty Attack mercilessly or just start casting normal spells like Magarudyne and hyper-focusing on cancelling buffs and debuffs. Or maybe Nyx's A.I. becomes too muddled because it just gains the capacity to use every skill in its arsenal at the very end. Those are just theories though.
I also think the glitch comically reduces the EXP for beating Nyx, because I swear it gave more than 24. At least I didn't have to worry about gaining another level at the last second raising my final level.
Persona 3 FES (Low Level, Hard) - Nyx Avatar