This is based on a Emerald emulator semi-nuzlocke (only semi because each Pokemon gets 3 lives) because it's been years since I beat Sapphire, Ruby, and Emerald (and I haven't reset them)
Lombre can crush Roxanne's team with Bullet Seed.
Brawly was my toughest because Makuhita got 5 Arm Thrust hits on my Combusken and used Bulk up a lot so Taillow had problems.
Wattson has an irritating Magneton unless you've got Combusken/Blaziken or a Makuhita/Breloom.
Flannery's Torkoal was vicious especially with that White Herb but after it KO'd a Swellow and Lombre Aron was able to crush it.
Norman was actually pretty easy I thought. I really smashed his team with Blaziken.
Winona has some definite type advantages in Dragon and Steel, but provided you have a solid Electric type her team's not too tough.
Tate & Liza I actually felt were laughably easy. Claydol was a bit tougher but the rest of their team went down fast and the battle became 2 on 1. R.I.P. Swellow

Juan...I don't actually know yet

I'm en route to Sootopolis atm. He should be easy from what I remember of him.