- Available: Reach level 55 with both Persona 4 Protagonist and Persona 3 Protagonist, then enter the Nurse's Office
- Rewards: Adamas Steel, 12000 EXP
Margaret wants to fight you, but only those with the power of the wild card, meaning it'll be just the two protagonists. The battleground will be the room near the end of Ch. 3 of You in Wonderland, south of the room with the three Painting Soldiers. Unlike other optional bosses, Margaret isn't as challenging, due to only having two party members. That said, it will still be a difficult battle due to the same fact.
One of the first things you want in this battle is the ability to inflict Magic Bind, so if you have Binding Hands as well on the Persona, it will be a lot easier to make it stick. For the first part of the battle, Margaret will likely stick to physical attacks, both a normal one and Vorpal Blade. She only gets one attack per round, but if you have an ability that can lower her strength, do that (Tarunda). Poison, coupled with Impure Reach, will also help whittle down her around 4,000 HP (it will hit around 650 HP each round, so it will help in making short work of her).
Once she gets down to around 25% HP is when you definitely want to have Magic Bind on her, as she will start spitting out mid-level party elemental skills, such as Magarula and Mazionga, with each protagonist having a weakness against one of them. Of course, you can always fuse a Persona that has the Resist element on them, although it might be easier to just get Magic Bind on Margaret. Considering she has no weaknesses, resistances (ailments and binds excluded) and doesn't null anything (besides the obvious light, dark and Down status), just hit her with your strongest attacks.