As of 19/11/14, there seems to be a visual bug with this mission where the second threat (Magic Debuff) isn't displayed in the mission details while still being active and therefore it looks as if you only need to counter Massive Strike which could cause confusion.
Just make sure you throw in a follower who can dispel.
Garrison Timer Mobile App
[Upon completion your 随从 will gain: ]
Guide to the Garrison Barracks
Missions and Followers math for maximising success chance
Warlords of Draenor Garrison Missions Guide