I have all 8 badges in Diamond and Platinum, so I'm listing Platinum's cuz it was way more recent.
1. Took Roark out easily with my Turtwig.
2. Gardenia was an issue, seeing as how all I had at the time was Luxio and Grotle. I beat her, though.
3. Fantina was simple. Grotle and Luxray knew Crunch, and Gardevoir knew Psychic for Gengar.
4. Maylene was simpla, as Crobat and Gardevoir just wailed on her Pokémon.
5. Wake was pretty easy. Luxray andTorterra did a lot.
6. Byron went down to Vaporeon, who took them on single-handedly. I was training Vaporeon in Iron Mountain to catch up with the rest of the crew.
7. Candice was problematic. Gabite, Crobat, and Torterra were next to useless this match. Gardevoir proved useful, though. Ironically, it was Crobat's Cross Poison that hit Abomosnow, her final Poké, and granting me victory.
8. Volkner was a cake walk for me. Garchomp and Torterra teamed up to take him down. Electivire's Ice Punch didn't even stop 'em.
There we go. :)