Hello! I am unsure if you need help anymore but I am willing to share with you the Shadow Yukiko strategy I scribbled in the back of my science notebook (because screw my science teacher).
Use the Persona Orboras (resists Yukiko's moves). Hit Shadow Yukiko with Bufu (Chie) 3 times. Then she'll put up White Wall, so wait another 3 turns. She'll summon the Prince when about half of her health is done, but he'll leave when only a quarter of her health is left, so don't bother attacking the Prince. He can silence Chie, but Chie is pretty strong and it only lasts a bit so it's fine. Heal your main character and Chie as soon as possible (like, if they're at 75% health HEAL THEM). Only really care about keeping your main character and Chie alive. Use any SP refilling items ONLY on Chie, don't use her status buffs/debuffs (at this early in the game it didn't help much). GUARD WITH YOUR LIFE WHEN YUKIKO GIVES "a passionate stare." If you're low on medicine, only revive Chie (Yosuke only matters becausd he has Dia). Shadow Yukiko will begin to use Shivering Rondo when she only has a quarter of her health left, and she will only use it on people who are inflicted with the Fear status. So if anyone is fearful, GUARD WITH YOUR LIFE. Use all out attacks as much as possible.
And that's how I beat Shadow Yukiko. Now, my strategy is kinda strange since I had to do this with little medicine and little time left, but I hope it helps!