Since Whitney's Miltank has a great defensive and knowing the move "Rollout", you'll need a Pokemon that knows a Fighting-type move and have a great Attack. Example, if you can teach a Croconaw as a starter a Fighting-type move and a good defensive, that will be OK. Or you can use some stratergy. If you choose Cyndaquil as a starter, make sure it knows the move Smokescreen. Use that move several times, the Miltank's accuracy will be lowered. When the Miltank's accuracy can't be lowered, use Ember. Ember has the chance to burn the opponent, and if you're lucky, you're able to burn it and it's HP and Attack will be lowered. Or if you catch a Mareep and evolves it, first use Thunder wave, then Thundershock it.