If you can trade-evolve and are willing to give up Eevee (or get insanely lucky), Alakazam does even better since it can learn Psychic before level 50 IIRC.
In other news, still grinding -_-
You can probably pull it off with Kadabra too, since iirc his SpAtt and Speed are comparable to Espeon's. He would just be a little more fragile. Plus it's tougher to get an Abra (unless you buy one from the Game Corner, which would require long hours of Voltorb flip) than an Eevee.
I mean really there's lots of options, some probably even better than Espeon, it just happened to work really well for me so I posted it. Karen is going to be rough though, let alone Lance. Especially with no restarts or healing, ugh. And I have to do it all with a L43 Crobat, L44 Feraligatr, and L45 Ho-oh. I have a feeling I'ma need some serious luck on my side, or maybe a sacrifice or two.
Does Nuzlocke include Kanto and Red, or just the E4? My strategy might change if this is the case.