...What? Since when?!? Teddie is arguably the most versatile party member in the game after the protagonist. He is the undisputed best source for buffs after the protagonist. He is the second best healer in the game and his good magic stat makes him a good backup attacker.
The argument that he's made redundant by Yukiko is completely absurd. The 2 characters work very well together; not only does Teddie resist her weakness, and his access to healing takes the pressure off Yukiko (IE, you run into situations where healing is needed and Yukiko's has already been used) but he is capable of focusing on his buffs due to Yukiko's access to healing. Teddie is a very solid character.
Seks, just run with Yosuke, Kanji and Teddie. Mind Charge / Agidyne spam with protag, Yosuke is on support (items, backup offense, etc), Kanji should spam Fatal End (I don't know why you were using Zionga, Kanji is a physical attacker) and heal with Teddie when needed. Grinding won't get you any stronger; you need to overcome Mitsuo with strategy.
PS: Don't bother with Yukiko now. She learns Mediarama at level 43, so it's simply out of your reach for now. Have her catch up in the next dungeon. You definitely fucked up when you dropped her for Kanji.