Get the Three Johto Starters.
To get the three Johto starters, you have to beat the 100 trainer Mt. Battle Challenge. When you do beat it, you get to pick ONE of the three Johto Starters. To get another you have to beat the 100 trainer Mt. Battle Challenge all over again. You have to beat it three times to get all three starters. It's worth it though, because they get the elemental hyper beams, frenzy plant, blast burn, and hydro cannon, which they can't get any other way but this.

Chikorita w/ frenzy plant --> Beat 100 trainer Mt. Battle Challenge
Cyndaquil w/ blast burn --> Beat 100 trainer Mt. Battle Challenge
Totodile w/ hydro cannon --> Beat 100 trainer Mt. Battle Challenge.