Jolteon was basically impossible to solo prior to Groudon because it's defence is too high. Winning requires you to bring enough DPS and only Groudon is viable realistically - prior to Groudon, 6 Mewtwos could in theory with some luck, and Rayquaza has higher neutral DPS than Mewtwo but even still, has a higher than 180s theoretical time.
Even with weather boost it's hard to take down. In sunny weather, the next best ground types after Groudon (Rhydon + Golem) still barely get it done in around 170s at level 40, which just shows how small the list of viable Pokemon is.
