Whitney's Miltank has been known as one of the hardest bosses of the entire franchise. So for this post, I want people to post the best strategies to beat Whitney's Miltank and win the Plain Badge. I'll go first:
First, I got Miltank to use Rollout, I think with a Pokémon weak to the move like Hoothoot. Then I brought out Rocky, the Onix you got from Kyle in an in-game trade. Due to type resistances, Rollout wouldn't do much damage. You also might want to have him learn Rock Smash to finish her off easily.