I played Moscow Battle 3 times and couldn't get the percent completion to go above 0%. Then I played Avalon once and it worked. I'm thinking it's one of those "complete in a single sortie" challenges.
Is anyone else seeing this? Can anyone confirm the wording in the Japanese version? Here's the English text:
Sortie in the "MiG-31B" aircraft and use the "machine gun" to destroy a total of 30 target(s).
Show us your skills and obtain rewards.
[ Required Game Mode ]
Online Co-Op Missions
(and for far future readers, I'm posting this early June 2015)