Fighting Chuck in Heartgold. He was the only gym leader in HG I had a problem with, mostly due to my lack of Flying/Psychic Pokémon.
The first time I tried, his Primeape Double-Teamed to the point where I couldn't hit it at all, and fainted half of my team. I finally got rid of it, and then his Poliwrath one-hit-killed my last pokémon. (Quilava)
About a dozen tries later, I decide to train my Flaafy and evolve it into an Ampharos. Then I go into the gym… Use all but my Ampharos to faint Primeape, and then get two critical hits on the Poliwrath, and then he missed on all of his attacks.
I must say I was delighted.