Make sure your first three Digimon are higher than level 60. Also make sure that you have three weak ones because your Digimon will grow a lot. In the "Final Battle " mission, once you have reached Darkmoon City and have gotten the Master ID, go through the portal. You will battle against Chief Julia and CGallantmonC. Defeat her to get receive 9,600 experience points, save the game after the intermission sequence. The Chaos Brain is very confusing and has some powerful Digimon. Make sure to have revivers. Go through Chaos Brain and you will see someone familiar -- Grimmon with Chrono Core. Soon Grimmon will digivolve into ChaosGrimmon. He will be at level 88. Your first three Digimon should all be Megas. Defeat him and another intermission sequence will start. ChaosGrimmon will digivolve into ExoGrimmon. He will be at level 99. No not defeat ExoGrimmon if you want your Digimon to keep growing. If you decide to do this, your Digimon will lose friendship but will gain 15,000 experience points each time you do this. When you lose to ExoGrimmon you will be at your house. From there, just repeat the trick and your Digimon should get strong.

A Farmer's Island is required for this trick. You can get the coupon for a free Farmer's Island after you win the tournament. Deposit the Digimon you want to train after you buy the Terrain Chart, which changes it to be able to give your Digimon more experience. The following are items that raise certain Digimon stats and what they are most useful for:

Fire Road: Dragon types
Ball: Beast types
Mini Shower: Aqua types
Flowers: InsectGrass types
Computer: Machine types
Birdhouse: Bird types
Phonograph: Holy types (like Coronamon)
Odd Pot: Dark types (like Lunamon)

You will also need a Tiny Field so that your Digimon does not become hungry. You can get these where you obtained your Farmer's Island, only select Wormmon instead of Kumamon. Put your Digimon(s) in the Farmer's Island and train it. When it is done training, let it rest. When you talk to it again it should have leveled up. Repeat this until it reaches the desired level. Note: This trick can level up your Digimon back to ultimate after the virus sneaks into your city.