Servine is the least useful Starter Pokémon against Roxie. She is, after all, a Poison-type specialist. Here are some useful Pokémon for beating her:
Magby is useful for taking down Whirlipede with its Fire attacks, but is only obtainable in Black 2. You can catch them in Virbank Complex.
Magnemite is also obtainable in Virbank Complex. It can tank hits from Koffing and Whirlipede for days, and Thundershock has a helpful chance to paralyze. Magnemite is also an A+ Pokémon that is useful in any game.
Tranquill gets STAB Air Cutter with a high crit chance, especially if it has the ability Super Luck. It also gets Roost to restore HP and Quick Attack for being speedy.
While Herdier doesn't have a specific type advantage over Koffing and Whirlipede, it has good bulk, as well as access to Intimidate and the deadly Silk Scarf Return, as well as Work Up to boost your Attack.
These Pokémon are probably your best chance of beating Roxie. If you fail the first time, don't worry, perseverance will triumph!
