2nd Gen Your Whitney Strategy

Be overleveled because Bugsy's Scyther has a type advantage over your Chikorita making him a far more threatening.

I traded in game to get a Machop and just kept being defeated by her until my Machop was leveled up enough to defeat her.

Reduce Miltank's accuracy with a smokescreen if you have a Quilava by that point. Using a sturdy fodder Pokemon to reduce its stats before it faints can work too.

Use Cindaquil/Qilava to smokescreen her to utterblindness, by her I mean milktank, then send in my machop that I traded for and kick her milk giving butt!

my personal strategy is thoughts and prayers.

All i did was trained my cyndaquil. i used it for everyone and ignored rest of my party.
Just kept a lvl35 weepingbell thinking it might come in handy for defeating rival.
I trained cyndaquil to lvl70 and at the 5th attempt defeated elite four and champion with only a single strong pokemon (others were 5 or 6 except weepingbell) (i was feeling way too much proud)

Use Female & flying pokemon that's my tactic