Let's talk about her in a Gold/Crystal speedrun for the funstuffs, since I wrote this up for whatever reason.
Now when you hear Whitney you might be thinking to yourself: Miltank, that'll be tough. Uh-uh. If things go well, that fat cow is all but free. The real problem is the little ball of shit that comes before it: CLEFAIRY.
Let's go over its moveset. Encore. This is generally good if it happens. You do the run with Totodile et al as your lone champion. It'll be using Rage and only Rage in this battle, and this will save you some menuing. Mimic. This basically never happens and is a wasted turn if it does, though you don't really want it to use the mimicked Rage either. Doubleslap. This is what you want to happen. Every hit of Doubleslap will boost Croconaw's Rage level by one stage, so what you're hoping for is a Doubleslap with a high number of hits. Metronome. Oh my god.
That fucking move is a roulette of pain and suffering. One wrong roll, and you're done, son. Some of the horrible things that Metronome can give you include: Thunder Wave, Zap Cannon, Dynamicpunch, Spore, Confuse Ray, Sacred Fire, Charm, Reflect, Sand Attack, Disable...so much can go wrong and do anything from waste horrendous amounts of time to outright killing the run. The worst are Selfdestruct and Explosion, both a cocktail of instant death, guaranteed. gg.
See, once you get the boosts from Rage, Miltank isn't so bad. You can just body the hell out of it with your pissed-off Croconaw and get the win. You may fear Rollout, but that's what you want! It won't build up enough power in time to kill you before you kill it. The only danger at this point is a flinch from Stomp or a critical Stomp, both of which are bad news. But without the boosts from Rage, you get wrecked no matter what you do.
You can see now why Metronome from Clefairy is such a big problem. Just not using Doubleslap is bad enough, and even though it can sometimes give you a substitute like Barrage, you're way more than likely to get something that will kill your ass. Just use your imagination. Even things you wouldn't expect like Baton Pass (switches to Miltank), Conversion2 (can't build up Rage anymore), Transform (loses Doubleslap), and Belly Drum (can cause Clefairy to die too early or make it do too much damage) are pretty much run over.
This is an absolutely brutal fight due to that one move, and is definitely the third-most obnoxious in the speedrun.