All right, so Jolteon and Thunderbolt aren't an option. What about Fissure? If you can control the outcome of any attack, instant kill moves like Horn Drill and Fissure should be able to connect every time. You get Fissure automatically after beating Viridian Gym, and the stats of the Pokemon who knows it shouldn't matter.
Just a suggestion-why not pick up Eevee and turn him into Jolteon? I would think it'd be faster, and I'd be very surprised if he couldn't use Thunderbolt to take out Lorelai almost as fast, even at a low level.
Actually, there's a couple of Fenix Downs in the Returners' Hideout. The closest one is probably the one in the treasure chest near the bed that Terra wakes up in.