Whitney is the Normal-type Gym Leader of Goldenrod City. At this gym, since there aren't usually wild Fighting-type Pokemon at this point, you will want to use your strongest Pokemon here, as her Miltank is quite tough.
-Her Clefairy is Level 17, and Female. This shouldn't be much of a challenge,
-Its Ability is Cute Charm, so watch out for becoming infatuated.
-It knows DoubleSlap, Encore, Mimic, and Metronome.
-Miltank is the powerhouse of Whitney's Team. Miltank could pose a threat to your team.
-Its Ability is Scrappy, meaning that its Normal-type moves are able to damage your Ghost-type Pokemon, that is, if you own a Ghost-type Pokemon. It is also holding a Lum Berry.
-It knows Rollout, Attract, Stomp, and Milk Drink. With its combination of Attract, decreasing your chance of hitting Miltank, and Milk Drink, replenishing its energy, Miltank can be very annoying.
For defeating Whitney, you will receive the Plain Badge and TM45, Attract. You will also be able to use HM04, Strength, outside of battle.