If you chose Agumon or Guilmon in the beginning, go on the "Treasure Hunt" sidequest set by the Digi-Elfs in the Central Area. Complete the dungeon, and bring back the Sword Cannon Delta. This must be done on Hard mode. If you chose Veemon in the beginning, complete Item Road after you complete Hard mode. If you chose Dorumon in the beginning, you can get him by going on the Booster Pack Collection sidequest set by the Digi-Elfs in the Terminal Area and completing the dungeon with only 1 HP, without recovering your HP or using a Gate Disk. This must be done on Hard mode.