Easy points:
Strike out all the batters in a two player game until the end of inning number nine. Get two outs and walk to the next four batters. In doing this you should earn enough points to buy 100 card packs (almost all of the cards available). You can repeat this process as much as you need to if you need more cards.
Late swing:
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.
Easy homeruns:
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.