There are a total of 13 pitch types in RBI 20. 9 of them are common in regular current MLB pitchers and 4 of them I could only find by going to Legends pitchers. You must use the Modern control setup for pitching to get this achievement.
First thing I would suggest is playing as the Cubs and selecting Yu Darvish as your starting pitcher. He has 8 of the 13 pitches you'll need. Throw each pitch type at least once using the various directions on the left stick, and then as soon as the game will allow you to sub him out, put in Kyle Ryan, one of the Cubs relief pitchers, who has a Sinker (SNK) and throw one of those. That should get you to around 69% on the tracker.
Next start up a game using the AL Legends for the last 4 pitch types.
Blyleven - EPH
Kaat - SCR
Fingers - FRK
Wilhelm - KNL
Throw each of those pitches once and finish out the minimum batters to be able to sub the next pitcher in and the achievement will be yours.
Just to give you a list of all the pitch types so you can check them off one by one as you go, here are the 13 pitch types in the game.