Whitney's Miltank is one of Whitney's Pokémon she uses during her gym matches. It also appears to be her most powerful Pokémon, giving Ash a run for his money in their first battle with its powerful Rollout attack. Miltank single handeley won the first match and caused Ash to rethink his battle strategy.
After Ash and Whitney managed to blast off Team Rocket from Whitney's uncle's Miltank farm, she decided to battle Ash again right there in a friendly match. In another 3-on-1 match Ash used the the dirt on the ground to trip up Miltank and even though Miltank took out Ash's Cyndaquil and Totodile, it was finally defeated by Ash's Pikachu, earning Ash the Plain Badge.