Identification
You encounter Shadow Teddie immediately after defeating Shadow Riser on the 11th floor of the Marukyu Striptease dungeon. Shadow Teddie emerges from a hole in the ground and appears as a giant, demented version of your friend Teddie. Shadow Teddie's favorite attacks include Mabufula, which inflicts moderate ice damage; Marakunda, which lowers opponents' defenses; and Foolish Whisper, which can silence all enemies. Strong and durable, Shadow Teddie can fight for an extended period of time unless you've upgraded the members of your party to level 30 or higher.
Personas
Shadow Teddie lacks an obvious weakness, so no specific persona will necessarily perform better than others when attacking him. However, Shadow Teddie inflicts significant damage with ice attacks, so have personas who possess immunity to ice. Jack Frost, King Frost, Ganga and Unicorn are four particularly valuable personas when confronting Shadow Teddie, as they act to eliminate a large chunk of his deadly arsenal.
Healing
All four members of your party should possess the highest possible health every turn in order to withstand Shadow Teddie's powerful attacks. The popular rich girl Yukiko, while somewhat of an offensive liability, owns exceptional healing skills, making her invaluable against Shadow Teddie. Have Yukiko use the Media support skill to heal the other members of the party on most every turn. Yosuke, the clumsy transfer student, may also serve as a secondary healer.
Defense
Shadow Teddie's strongest attack is his Nihil Hand, but it also provides your best opportunity to inflict damage. Shadow Teddie requires two turns to charge up the attack. Take advantage of his charging on that first turn to launch your strongest assaults. Then, on the second turn, make everyone in your party guard. As long as you guard, the Nihil Hand doesn't cause any damage. If you fail to guard, Shadow Teddie's Nihil Hand may render an automatic game over. Continue to employ guards, frequent healing and timely attacks to eventually wear down and defeat Shadow Teddie.