Pick a starter. Then, defend against the Caterpie Army (which is very easy). If a Caterpie somehow gets through, you lose 1 health. Be careful on where you place your Pokemon, you can't move them again. You can't place them on the path. There is a not so secret way to get multiple starters that was TOTALLY not inspired by a code error. Also, once you beat the Caterpie Army, stop the game, the incomplete Pidgey Army will likely make your flash crash.
Working on the Pidgey Army. Like that's going to happen...