I love using Gameshark to grab the pokemon. I just catch them at level 3, buy 100 master balls, and capture them. Then I can level them up from infants to the powerhouses they should be. Although cheating, it helps later on when your pokemon will have higher stats in the wild. However, some like Venomoth and Exeggcute will not learn skills for a long time.
To level them up, I either A) play the game normally or B) activate the fight this pokemon all the time and use Chansey.
At level 3, Chansey gives 109 experience
At level 20, 1000 exp O_O!
Chansey isn't hard early on, it may have high hp but a crap defense. Just watch out for that double slap if you do it.