Start a two-player game and proceed to strike out every batter on each side until the bottom of the ninth inning.
When you've got two outs at the bottom of the ninth, walk the next four hitters to earn enough points to buy 100 card packs, which should get you almost every card. If not, repeat and get yourself more card packs.
NOTE: This doesn't include special cards earned in Scenario Mode.
When you are at bat, for some reason when you press Swing, the player will actually swing a moment after it is pressed resulting in a swing and a miss. You should press Swing in advance, preferably after the pitcher releases the ball.
When controlling Barry Bonds at bat, press the Square so that you are on "Power" and not "Contact". When you are on "Power", only the yellow box will appear, and when you are on "Contact", there will be a shaded area covering the yellow box. If the pitch is up and in, move the box there and swing. It will almost always be an automatic homerun.
If you are down in the count or need a quick ball, pretend to lay down a bunt. Pull back while the pitcher is in his motion. This should result in a ball being thrown.
Note: This only works up to two times in an at bat
When your behind in the game because of a really tough pitcher such as Johnson or Martinez...after hitting a lucky homerun, press L3 or the analog sticks to taught the pitcher this will make him throw you more pitches at which to crush over the wall....sooner or later he will be subsituted for a pitcher who can't throw breaking balls...