I present to you not only a viable in game Pokemon, but a viable in game BUG type Pokemon!
With typing alone, it already has the advantage against the Bug, Fighting, Grass, Psychic and Fairy gyms, as well as Diantha's Gardevior, but with coverage it also has the advantage against the other gyms and two of the elite four types as well. Who could accomplish such a feat and still be accessible before the second gym and evolve early? Scolipede of course!
Scolipede @ Insect Plate/Black Sludge
Ability: Swarm (Speed Boost if available)
EVs: Attack and Speed
- Poison Jab
- Rock Slide/Swords Dance
Although scolipede lacks strong defenses, its high speed and attacking prowess more than make up for it, and it can even learn Swords Dance to boost its already solid power, boosted from base 90 to base 100.
Scolipede may be obtained as a Venipede on Route 6, where it will jump out of bushes to attack you.