.

Keeping this in consideration, how do you beat Whitney Miltank?

Method 1 Using Muscle the Machop

1. Go to the top floor of the Goldenrod Department Store, where you'll find a man who wants to trade you his Machop, Muscle. He'll ask for Drowzee in Gold and Silver, and Abra in Crystal.
2. Level Muscle to at least level 18.
3. Use its Low Kick and Karate Chop attacks against the Miltank.

Also, why is Whitney so hard Pokemon? And Rollout can wreck a whole team, no matter how many super potions you bought. However, the main reason Whitney is considered to one of the hardest fights in all of the Pokemon games is this: You don't have revives. Later in the same game, you meet Red who is ridiculously overleveled and has good type spreading.

Beside above, where do I go after beating Whitney?

After you beat Whitney, go to the building in the northeast corner of the city and talk to the girl there to get a free Squirtbottle. As you leave town to the north, talk to the guard in the guard house. He will ask you to deliver a message to the guy standing next to the Bitter Berry tree on Route 31 (remember him?).

What is the weakness of Miltank?

Fighting