Theres another way to get wins if you have friends online at the same time. This is particularly useful when you need to complete missions that require you to use some bad ninja that would never win an actual fight.

At most there can only be 1 person waiting in the Training queue. What you need to do is have 2 other friends online. Queue in series, with the person that needs to complete the win missions in the middle (Player #2). If someone is waiting, Player #1 will flush him out, so Player #2 and #3 can meet and give #2 the win. Else, Player #1 and #2 will meet and you can get the win there.