KING - 20
Beat all teams in CHALLENGE MODE
From the Main Menu, select Challenge Mode. You must beat all 30 MLB Teams in Challenge Mode for this Achievement to unlock. Unfortunately you cannot lower the innings in Challenge Mode, so this Achievement will take some time as each game is a minimum of 9-innings.
I was able to win the first challenge against the Pirates in 3 innings. Of course, I haven't played any other games in Challenge Mode but possibly they don't have to be 9 innings. I'll check again, though.