>>53537131I lost to Misty twice a few weeks ago when replaying RB. All 6 of my Pokemon were at lower levels than her, since I had Magikarp at the top of my team list acting like an experience sponge since Mt. Moon. Also didn't actively grind, as in I didn't just walk around in tall grass for the sake of battling, just fought all trainers and whatever wild Pokemon appeared naturally as I played through the game. Only thing I had actually strong against her was Pikachu, but again, underleveled and kinda sucks at that level range + point in the game.
One time it was really close and I got her Starmie down to literally 1 HP before it killed me. The other time it pretty much swept my team.
Eventually realized my levels were just too low for it and got some experience from the trainers on the route before Vermilion and evolved my Pidgey, which I think made some difference. And yeah I realize I might have set myself up for that with how I'd been training and that she might not be legitimately hard, but still.