Always pay attention to the levels of the monsters you are facing vs your level for a good indication of difficulty, and keep in mind that the easier the monsters, the less XP you get (all the way down to 0). Another good indication is if all the monsters are going down in one hit and you are not taking damage. At level 20, you are ready to go on to Act III. The only experience you might get from the battlefields at this point is by completing some of the merchant quests. Otherwise, grab any items you want to buy and move on.