Exp Grinding for Nat Dex

I'm trying to complete my national dex, but since I have a bunch of pokemon that need a lot of levels, what method is the faster of the 2?

Training the pokemon specifically with a lucky egg against wild pokemon it's level, or slapping an exp share on it and running through the league a million times.

Trying to figure out just to so I can be efficient about this grind, and would love to hear what others who completed their dexes did in these case.