How are the followers supposed to defate them :p
This mission (and perhaps all training missions) is available even if all of your followers are level 100, just rarely. Just like
, which I seem to get every day.
Also, this mission, like most training missions, has a 100% chance of success and mission rewards, regardless of which follower you assign to it.
Garrison Timer Mobile App
[Upon completion your 随从 will gain: ]
Missions and Followers math for maximising success chance
Warlords of Draenor Follower Guide
Warlords of Draenor Garrison Missions Guide