Attract didn't pose an issue for me, since I had my Level 24, female Quilava handle the entire battle. It was weak against Rollout, but it had also just learned Defense Curl.
While Clefairy (who I didn't consider a threat compared to Miltank) was out, I used Defense Curl over and over to build up an armor defense in preparation for Miltank and its super-effective Rollout. When Miltank came out, I fired one Smokescreen after another at it. Thanks to Defense Curl, Rollout and Stomp did barely anything, and thanks to Smokescreen, nearly all of Miltank's attacks missed. I eventually started laying on the Flame Wheel, and ended up burning Miltank twice (the first time, its Lum Berry healed it).
I thought I had it in the bag there, but then came the Milk Drink. That just made the battle drag on and on, and I was afraid I'd have to wait until Miltank ran out of PP for that move. Luckily, though, I got a critical hit at just the right time, knocking Miltank out before it could heal itself again.
Overall, Whitney was annoying, but not too much of a problem. Morty, on the other hand…