You must play Quick Ranked matches to earn this Achievement. That makes it a little trickier for boosting, since you can't see your opponent's Gamertag until you are in the match. I suggest joining a Party with your boosting partner and ensure that you are matched with each other (based on team selection) before you start the match. If you don't have a headset, arrange to always pick the same two teams against each other - it's cumbersome but effective. (If somebody can confirm that Quick Player matches will also work, please update me in the comments. We know that Quick Friend matches don't.)
After throwing the first pitch, have your opponent select Quit from the pause menu. Respond to the prompt that you do not consent to let your opponent quit the match (choose "NO"), and have your opponent respond to the following prompt that he wants to quit anyways (choose "Yes"). You will get credit for the win.
Repeat this step fifty times. It would be nice if you alternate wins with your boosting partner until you both reach 50. It takes roughly a minute and a half to match, load, pitch, and quit each game, barring any connection issues. So be prepared to set aside a few hours or to knock them all out of the way for the pair of you.
You should also plan your session to pick up the 3- and 5-game win streak Achievements along the way. You'll also nab Achievements for 10 online wins and 20 online wins en route to this bad boy.