Alright, I can help with these. I'm pretty heavy into SMT.
Minotaur's main problem is the War Cry debuff, which he then follows up with the Labrys Strike. War Cry's pretty lethal to the point where it wouldn't be out of place on a boss closer to the endgame, which is just terrible. That said, you'll likely want to abuse the Press Turn mechanic by peppering him with Bufu and using any turns that someone without Bufu may have on either buffing (mainly to either offset the War Cry or to boost Agility so that he misses more and loses Press Turns) or passing to someone who can cast more Bufu.
Word of advice, though, after this fight; overhaul your team to outfit them with Zan skills (with Zio or Gun resistance, if at all possible), or come packing lots of Zanma Stones. Medusa comes pretty quickly after Minotaur, switches up resistances and may well be just as hard because her Gun attacks hurt like a bitch.
As for Shadow Yukiko, it all depends on which version you're playing. If it's the original, she's much harder due to not having any glaring weaknesses and forcing you to essentially coddle Chie through the entire fight. If she or one of your Personas has Mabufu after she summons her first mook, you can use it to get a One More chance and deal damage to Yukiko at the same time.
In Golden she's much easier because now Shadow Yukiko is the one who's weak to ice, not her Prince Charming, so you can initially spam Bufu with impunity for All-Out Attacks and just keep Yosuke using either Sukukaja buffs, minor healing or some lesser damage. Of course, the next major boss right after is Shadow Kanji, and he's even more of a pain due to his two mooks... but worry about that later. If all else fails, grind grind grind.
Also, if you can find a copy of Devil Survivor 2 somewhere, I highly recommend it. They fixed a lot of the gameplay issues that were in the first game - it's much easier to level up your full party now due to leaked EXP, there's more demons to fuse, more bonus bosses, you actually have to earn your bonuses as opposed to the original throwing them at you all at once... Plus the characters are actually more interesting, too.