The Margaret fight is only available on New Game +
You’ll also need to:

• Complete Margaret’s social link
  
• Defeat all six optional bosses.

Immediately after obtaining the Orb of Sight, return to the Velvet Room and Margaret will issue her challenge. Do not progress the story until you've talked to her or you will miss your chance!

When you’re ready to face her, head to the 10th floor of Heaven.

You’re gonna want Yoshitsune (Tower 5-way fusion) with as many absorb abilities as you can scrounge up. Absorb skill cards can be obtained from:

• Wind – Horus (Sun)
  
• Electricity – Lucifer (Judgement)
  
• Ice – Isis (Empress)
  
• Fire – Hell Biker (Hanged Man)

Margaret does not use Hama/Mudo skills so you do not need Null Light/Dark.

If (like yours truly) you didn’t think about this before your very last day in Inaba, you can also obtain absorbs from level 10 skill cards, which drop during Shuffle Time in Yomotsu Hirasaka or (rarely) Magatsu Inaba.
Shuffle Time is the only way to obtain Absorb Physical.

The biggest issue is Yoshitsune’s innate Repel Electricity, which will give Margaret free heals. You will either need to overwrite it with Absorb Electricity or bring a party member weak to electricity (Yosuke or Teddie) so she’ll target them instead. (But seriously, don’t bring Teddie.)


You should also consider making a persona with Auto-Masuku, Auto-Maraku, and Auto-Mataru. Equip it before battle then switch to Yoshitsune on your first turn.

Those skill cards can be obtained from:

• Auto-Masuku - Kingu (Aeon)
  
• Auto-Maraku - Setanta (Emperor)
  
• Auto-Mataru - Narasimha (Sun)

... or from level 9 cards during Shuffle Time.

First, it’s worth considering if you even want a full party - the more allies you bring, the less mileage you’ll get out of absorb heals (see below). On the other hand, depending on difficulty level and how much time you’re willing to spend juking Yoshitsune’s stats, you may have trouble pulling enough damage if you go it alone.


Your top choices are Kanji and Chie. For the third, Yukiko with Mind Charge can pull the most damage. If she doesn't, consider Yosuke for his absurdly high evasion - use Youthful Wind/item heal as necessary.

• Chie's max SP is 298. Dragon Hustle costs 150 SP. Equip her with a Chakra Ring (halves SP cost), and suddenly she can get off three casts instead of just the one.
  
• Rudra Ring (halves the cost of physical skills) is a solid choice for Kanji.
  
• If Yukiko/Yosuke already have their respective Boost and Amp skills, "Vow" accessories will not increase their damage any further.

Also of note: it is possible to down Margaret with melee criticals.

Phase 1: The fight starts out soft, with Margaret dealing not-very-impressive melee attacks to a random party member. Like the reaper, she gets two attacks every turn. After a few rounds, she’ll bust out the persona compendium.

Phase 2: Margaret will cast four elemental attacks in this order:

1. Fire (Siegfried - Ragnarok/Prominence)
   
2. Ice (Loki - Niflheim/Cocytus Pain)
   
3. Electricity (Okuninushi - Thunder Reign/Jihad)
   
4. Wind (Cu Chulainn - Panta Rhei/Divine Vacuum)

• If a party members is weak to a given element, she will ALWAYS cast the single target version on them.
  
• If no party member is weak to that element, she will use the AOE version.
  
• If she successfully knocks someone down with the single target skill, on her extra turn she will usually cast the AOE version of the same element, but may go to the next in the cycle instead.

Because the order is static, you can just have your weak party member guard for that turn (or, if their health is ok, take your chances on an evade.)

• After one cycle, she will cast Mind Charge > Megidolaon (Ardha), or Power Charge > Hassou Tobi (Yoshitsune).

In the early parts of the fight, she will mostly use Ardha, but occasionally swap in Yoshitsune. The Megidolaon can be rough on the Hero, so it’s important to keep your defense buffed. As the player’s Yoshitsune has innate null physical, the Hassou Tobi is no risk to you, but can easily do in your party members.


Phase 3: At a little under half health, she will pause to tell you how great you’re doing… and then cast Diarahan. Fortunately, this only happens once. This will be followed by Morning Star (Helel). She will then restart from phase 1, except she’ll start using Yoshitsune more often than Ardha.

Every 25 turns or so, Margaret will have Oberon cast Megidolaon. It will wipe you.

This effectively creates a time limit based on how many survival/auto-rez abilities you have. Rise at max Social Link has two. Other options are the Enduring Soul ability and the Patient Collar accessory. If you activate a barrier ability (e.g. Naoto’s Shield of Justice), Margaret will simply cast something else first to pop the shield and then the Megidolaon.

Rise Mechanics:

• The first will activate when any party member takes a fatal blow, so if you’re not careful, you may waste it long before the Megidolaon. (It will appear as a barrier with the message "Rise blocked all attacks".)
  
• Her second rez will only affect the Hero.

If you are relying solely on Rise, that will give you 75 turns in which to beat Margaret.

Hero: Power Charge > Hassou Tobi.
Heat Riser is only necessary if you're not using other party members' buffs.
Make sure not to waste your Hassou Tobi:

• After Mind Charge (Oberon), as she will be immune to all damage (except almighty)
  
• After Margaret's Hassou Tobi, as her Yoshitsune will absorb physical damage.

Allies:

Chie: Dragon Hustle (Masukukaja, Marakukaja, and Matarukaja in one) every three turns.
Otherwise, Power Charge > God’s Hand. Defend on Fire turn.

Kanji: Power Charge > Primal Force. Defend on Wind turn.

• Consider popping off a Ziodyne when Rise’s given everybody Mind Charge.

Yukiko: Mind Charge (if she has it) > Agidyne/Burning Petals (depending on SP). Defend on Ice turn. Heal as needed.

Yosuke: Youthful Wind (a combined Masukujaka/Mediarama) or item heal as needed.
Otherwise, Garudyne/Brave Blade.

• Don’t bother defending on Electricity unless the extra damage would wipe the party.

When using elemental skills, it’s a good idea to Analyze each turn to make sure you aren’t healing Margaret.

Remember that the Rise buff "All allies are now charged and focused" gives each party member a free Power and Mind Charge.


Good luck against Margaret!