Instructions
Recruit Solarmon. You can find Solarmon on Proxy Island, which you must visit during Chapter 4-10 of the main story.
Put Solarmon into your party, or train him at the Digifarm, to have him Digivolve into Clockmon. Solarmon must be level 33 or higher, with an Aptitude level of at least 40 and 6,000 or more points of Holy EXP, before he will Digivolve.
Train Clockmon until he is level 40 and has an Attack of 200 or higher and a Defense of 180 or higher. When you meet these requirements, Clockmon will Digivolve into Knightmon.
Use Knightmon in battles to have Clockmon Digivolve into Alphamon. When Knightmon reaches level 65 and has at least 50,000 Holy EXP, he will Digivolve into Alphamon. Reaching 50,000 Holy EXP is best done by training at the Digifarm with the Gramophone, Holy Bell and Stone Henge farm goods, which you can purchase at shops throughout the game.