Whitney, for me, is one of the most anticipated gym leaders in all the game...
Debatably, I find that Whitney is the most difficult gym leader to defeat in any of the Pokémon game releases. Coming to defeat her often takes a single strategy. Some people find that winging it works better than strategizing her defeat, but in most cases, once you lose to her you realize that you need to do something different with the way you handeled the battle. And it isn't the battle itself that's difficult; it's her Miltank.
As for my strategy defeating it - at first I thought it was to overpower it. So I faught it thinking nothing of it... and I lost. I didn't want to let that happen again, so I took advantage of my Flaaffy's static ability and used that as my key towards defeating her. With the chances of paralyzing her Miltank, I combined that strategy with overhauling X stats; special and speed. I did so in my battle with her Clefairy, which is what I believe was put there for the purpose of preparing for Miltank. Other than that, it's pretty pointless of a Pokémon.
I took the time to use my X special to be sure that my ThunderShock would do the trick. I used about six or seven of those and then took down the Clefairy. When Miltank came out, lucky me, it attacked and got hit with that static ability on the spot. I was surprised because of how lucky I was for that. Then, I used ThunderShock and it... fainted. Hell yeah! I think I managed to defeat it so easily based on luck, though. Thankfully, my Flaaffy was a female and the attract attack wouldn't be able to do anything against it. I think if it did that, I would have been done for considering most of the time attract makes it so I can't attack at all since I'm somehow ALWAYS immobilized by love.