At the end of Digimon World 3, after defeating MAGAMI's President, seeing 3 Vemmon in Asuka's A.o.A Headquarters and finding out most humans have been turned into Oinkmon in both servers, you must go to Amaterasu City. In Amaterasu's A.o.A. Headquarters a strange machine appears, calling itself Lord Megadeth, which will lead you to the Gunslinger Spy Satellite in order to battle you. As you arrive, you will see that there are several Blocks blocking your way, you have to teleport around to find the machines used to move the Blocks (2 of these machines are guarded by a robot with an Infermon and an Apokarimon), and later go to the Second Floor of the Satellite. However, as you go to the Control Room, there's an earthquake and an Armageddemon appears. You must defeat him, and then you face Lord Megadeth.
Finally, after defeating Lord Megadeth, it will leave to awaken Snatchmon, and you must follow. However, Snatchmon refuses to obey his master; he instead absorbs Lord Megadeath's Matrix Data. As he states he'll fuse with the Gunslinger to become Galacticmon, and then with the Earth to become Gaiamon, Junior opposes him, and Snatchmon agrees to have a final battle against Junior as Galacticmon.
Galacticmon has two parts to attack you: the tail and the head. The tail uses only physical attacks. Immediately after defeating the tail, the body appears and uses Ragnarok Cannon, which reduces the HP of your Digimon to 1; then you have to face the head. It is much harder to defeat than the tail, as it is much faster, has much more HP, can Counter attack if your attack missed, takes lower damage, and also copies one of the techniques used in the battle against the tail. Once you finally defeat Galacticmon's head, the A.o.A is absolutely defeated.