You'll want to do this after beating all the events in Hero Mode. If you've beaten Hero Mode, you'll have access to all the battles you've done before. Each of the stars on the map are event battles, and some give you more AP for winning than others. The most efficient training method that I found was to beat Captain Ginyu (the star at the Wasteland) repeatedly, since he's very easy to beat and gives about 50 AP each time. You can skip the story dialogue fairly quickly by repeatedly pressing the button and then the button.
If you start to get tired of beating him, you can always do another story battle for a little while. Piccolo (the star at the Glacier) is a good one as he gives about the same amount of points. He takes much longer to beat though. You can also alternate between doing the event fights and working on the training menu to help relieve your boredom. Just keep at it until the achievement unlocks. If you're using a Balanced Hero, this should be around the time that his skills reach the A rank.