After you defeat Leomon in the first dungeon, you can return and fight a level 5 Gazimon and a level 5 Gizamon. After defeating these, you gain 39 experience points and 260 bits. Note: Only use this trick when you have at least one or two champion Digimon, as they are quite difficult to defeat.
Use the following trick to level up after one battle. Later in the game when you get two champion Digimon, catch a Rookie Digimon and put it in your party. Go to the Boot domain and fight the level 5 Gizamon and Gazimon. Your rookie Digimon should level up. When you check your rookie Digimon's status it should say 0 exp. to the next level. This will stop working when the exp. grows out, as in level 8, but the stronger the Digimon you fight, the longer you the longer your exp. stays at 0.