The double battle at the end of Victory Road in Emerald with skill swap Gardevoir and Slaking. Easily the hardest NPC battle, especially when all your Pokemon are at red HP/PP and you're rushing to heal up from all the other trainer battles in Victory Road.
Of course, you can fight them one by one, but there's no fun in that!
Machamp used Cross Chop.
It's super effective!
Weavile lost 2222% of its health.