I think I will help you out.
In Pokemon Black and White the eight gym will be a Dragon type gym facing Iris(White)/Drayden(Black).
However, both of them have same Pokémon.
They will have
Also both will have the movesets too.
Every mon has Dragon Tail, a Dark-Type move (Assurance/Night Slash), Fraxure will only have two dragon type move (Dragon rage including Dragon Tail).
With an ice type move, it will be easy to handle this Gym. Lv.65 will be perfect with Quiet Nature and MAX SpA.
TM Blizzard is useful and easy to get but its accuracy is a concern. You can Blizzard for best results.Blizzard can be obtained in the Icirrus city for 70000 Pokedollars.
This set can clear the Gym easily.
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