The final battle mission can be taken if you have completed the mission to defeat kuwagamon, meramon, monochromon and leomon in Infinite mountain dungeon. The way to complete the Final Battle Mission in Infinite Mountain is to buy angelmon feather from the Angemon NPC in Infinite Mountain. After the mission is complete take the mission from the white-beard guru near the elecmon (the entrance to the Infinite Mountain Dungeon).
Then go to the white beard guru in FIW *it’s below near the monochromon spawn place).after the mission is complete, a ball will appear which is a portal to ServerContinenDesert.