I currently have this mission in my Scouting Map; however, I do not have any champions who can use the Blizzard ability to counter the Legion Endbringer minion. I'm unsure of what to do next to acquire a champion for this mission, so any help that can be provided will be greatly appreciated.
I could not make the mission 100%. I waited for my Tirisgarde Apprentice troop order to finish and just tried again. It brings the mission to 101%! They are acquired by Grand Conjurer Mimic across the hall from the Training Dummies.
Garrison Timer Mobile App
Legion Class Hall Mission Guide