I think I will help you out.
In Pokemon Black and White the eighth Gym is a Dragon type gym facing Iris (White)/Drayden (Black). However, both of them have same Pokémon.
They will have
Fraxure (Lv.41)
Druddigon (Lv.41)
Haxorus (Lv.43)
Every mon has Dragon Tail and a Dark type move (Assurance/Night Slash). Fraxure has two Dragon type moves: Dragon Tail and Dragon Rage.
With an Ice type move, it will be easy to handle this Gym. Lv.65 will be perfect with a Quiet Nature and max Special Attack.
TM Blizzard is useful and easy to get, but its accuracy is a concern. Blizzard can be obtained in the Icirrus City Pokemon Center for 70000 Pokedollars.
So,
Oshawott @ anything
Ability: Torrent
Quiet Nature
Level: 65
- X-Scissor
- Revenge
- Blizzard
- Surf
This set can clear the Gym easily.
Some calculations:
Lvl 65 252+ SpA Oshawott Blizzard vs. Lvl 41 0 HP / 4 SpD Eviolite Fraxure: 194-230 (165.8 - 196.5%) -- guaranteed OHKO
Lvl 65 252+ SpA Oshawott Blizzard vs. Lvl 41 252 HP / 0 SpD Druddigon: 188-222 (123.6 - 146%) -- guaranteed OHKO
Lvl 65 252+ SpA Oshawott Blizzard vs. Lvl 43 0 HP / 4 SpD Haxorus: 218-258 (166.4 - 196.9%) -- guaranteed OHKO
Hope I Helped