I think this was against the Elite Four in HeartGold, the second time around. I was heavily under-leveled, and I was just using them to level up my team, in particular a Dratini I had wanted to evolve. Since I was just levelling up my team, I was just fighting against them repeatedly for experience, and I certainly had no expectations to beat the E4.
At one point, I got up to Koga's Muk, who had (probably) +6 evasion due to Minimize and had managed to Toxic stall my entire team... except for said Dratini. Muk was Koga's last Pokemon by then, and had taken considerable damage, but still managed to avoid enough attacks to eliminate the rest of my team (or at least whoever had survived until then - I wasn't using items at the time).
What happened was interesting, to say the least. I recall it hinged on Dratini being able to hit Muk without running out of PP, Dratini not hurting itself from Swagger and Shed Skin curing Toxic whenever applicable. It was quite a long-winded battle, especially considering Muk's Black Sludge, but I do remember Dratini dealing the final blow with ExtremeSpeed while he was at red health.
Of course, I didn't get very far against Bruno with a weakened Dratini (he may have evolved at that point, but I honestly can't remember), but it was still quite a feat for me to have survived the battle against Koga, haha.