Jolteon solo
Partly cloudy, Thunder. 10-12 sec left.
Two Groudon at level 30 is basically what you need. Took out 75-80% of HP and then finish off with whatever high-DPS that you have. I’ve only got rock-Rhypherior so far, so couldn’t test it. Simulations say that even a level 20 Groudon can manage after the two Groudons.

Answers
Pre-Groudon the only theoretical means of soloing Jolteon was by using a team of six maxed Shadow Ball Mewtwo. To my knowledge there's no record of anyone accomplishing this.
EDIT: To elaborate on this, this is a chart of the top DPS counters with Gen 3 and later filtered out until only what was available at the time of the first raids was left. Keep in mind the DPS numbers were likely lower as well since it was pre-stat rework. 16.67 is the cut-off DPS needed to deal 3000 damage in 180 seconds.
Nice! I've got OCD about leaving Pokemon non-maxed so I try to power everything I want to use up to 40, and did this raid (no weather boost) with two Rayquazas, one Groudon and one Shadow Ball Mewtwo. Groudon was clearly the star player even though the Rays pretty much match him for DPS since he just won't go down. Even though I managed with plenty time to spare, this convinced me to power a second Groudon up to 40. I'll probably wait for his eventual return and then max the best one. (My current best non-maxed is 91% with 15 ATK.)