Or against the computer on Rookie. All very simply, I've completed the Harper one already in just two games and about 4k on crappy bronze stadiums. The Moncada one is a bit of a beast though considering how bad the reward is.
You need 10 future stars players, you need 200 IP with them and 600 at bats, along with beating the White Sox on HoF and a couple of extra missions that are hidden until you unlock the players at 50 and 75%.