It's a tie between Clay and Elesa due to Excadrill and Volt Switch until Black 2. Excadrill was surprisingly taken down with a combination of Leech Seed and my Huge Power Azumarill, who was 8 levels lower than it was and For Elesa, a combination of Stealth Rock from Dwebble and my Male Audino's Attract and Echoed Voice combo. Drayden was surprisingly pretty easy in Black 2 as well thanks to my Aggron holding a Rocky Helmet.
Trainer Name: Heather
Friend Code: 1247-1004-2423
First D/P Shiny: Modest Female Magby holding Magmarizer (No PokeRadar)