Talk to Renamon in Shine Plaza. You learn that three Dorugamons are digging the Login Mountain to the point the mountain could collapse. Go to Login Mountain. You start from a different portal. That doesn't stop you from knowing the entire map. When you find a Dorugamon, it wants to dig and is going to put up a fight if we're to interrupt. So fight a mini boss battle. Do you remember there being three Dorugamons? So find the other two. Report back to Renamon and collect your reward.