To easily gain experience, you will need to have your first three Digimon at level 60 or higher, and have three weak ones because your Digimon will grow a lot. After reaching Sunshine City and getting the Master ID int he "Final Battle" mission, enter the portal to battle an OphanimonC. You will gain 9,600 experience points by defeating her. Save the game after the intermission sequence. The Chaos Brain has some powerful Digimon. Make sure to have revivers. Go through Chaos Brain, and you will see Grimmon with Chrono Core. Grimmon will soon digivolve into ChaosGrimmon and 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. Do 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. Repeat as desired.