I've taken a level 100 Magikarp through the Elite Four, with the aide of only one other Pokemon. Granted, that Pokemon was a hacked Spiritomb with Wonder Guard that I got over the GTS, but I felt I had to at least use them a bit before I release them both (and now they're in the Hall of Fame forever!).
Other than that, I have used Butterfree as my main Pokemon numerous times against the Elite Four, which stat wise, is worse than a lot of middle evolutions of Pokemon let alone final ones.