A bit cheap, but this is how I recommend doing it:
* Edit one team to have all players with max stats. This is your team.
* Edit another team to have all players with zero stats. This is the AI opponent team.
* Enter in the cheats 'triplecrown' (improved hitting for your 3-4-5 hitters) and 'm4murder' (improved power for your team).
* Change controls to use the "Classic Control" for hitting.
* When swinging, hold up on the Left Stick. This will prompt you to hit fly balls.
Doing this, I hit all kinds of home runs and scored over 20 runs by the 5th or 6th inning.
You should note that there is a roster save bug in this game. I recommend downloading the free Olympic Stadium DLC before attempting to edit rosters, as this DLC apparently includes a fix that allows you to freely edit and save rosters without errors.