She was pretty tough... I managed her Clefairy with Pidgeotto and Gust, with a few Quick Attacks and Tackles thrown in, but that Miltank was infuriating.
I sent out Flaafy and paralyzed it, and then ThunderShock/Tackled until it was down in the yellow... then it used Milk Drink. -.-
So I kept going, got it about halfway down, it Attracted my Flaafy. So I switched to Croconaw and it used Milk Drink. -.-
Miltank proceeded to Stomp my Croconaw down to nothing, all the while healing itself with Milk Drink. -.-
Out goes Flaafy, and I used a Revive on Croconaw, got it down in the red with Flaafy, and it was paralyzed so it couldn't use Milk Drink. I thought victory was mine, and what happens?
Whitney uses a Super Potion. -.-
So I kept going with my Flaafy until he gets knocked out with Stomp. Out goes Croconaw.
Two more rounds of getting it low with Ice Fang and it using Milk Drink later, I'm safe in the knowledge that she won't use another Super Potion on it [because they only ever use one, I believe] and wouldn't you know it, it gets paralysed in the red!
One Ice Fang later, and Croconaw is victorious!
Note that this took me about 40 mins. That's a tough victory!