Don't know if it will work, but your best chance would be Diglett with Mud-Slap. You could level it up a bit before surpassing the IV of a Jolteon.
Just ran it through a battle sim.
A 15ATK/0/0 L1 Jolteon Defender has 24hp and 35CP.
A 15ATK/0/0 L2 Diglett has 14CP, and will do 13dmg with each mud-slap. So, Jolty would be dead in two hits for 1000XP.
Timing will be tough. Mud slap is a 1.4sec move, and VS is a 2.3 sec move, so you are 0.5 seconds short of it being safe. You would just have to hope the first attack is delayed by half a second (I believe it is), and you have absolutely no lag.
May work.... Might not... No guarantees... You would have to try it...
