I'm late to the Legion Class Hall game, but let me share something that might be valuable to others.
By the time I received this quest, I had obtained 6 of 5-allowed Champions: Vindicator Boros, Justicar Julia Celeste, Lady Liadrin, Lord Maxwell Tyrosus, Delas Moonfang, and Aponi Brightmane.
When equipping my Champions to do these 6 missions, I was not able to click "start mission" and got the error that I needed to *deactivate* one Champion before I could continue, saying I could not have more than 5 Champions active.
Going to my Follower tab and choosing to deactivate a Champion pops up a message saying "Are you sure you want to deactivate *so-and-so"? Reactivating a follower costs 250 gold.
So you're telling me I can't keep all the followers you have given me *active*, AND I have to PAY to reactivate the one I deactivate once I get to the Order Hall Achievement for x-amount of followers? Thanks a lot! Goblineering with "best deals anywhere" at it's finest. :-/
Not knowing which one to deactivate, I researched each of the Lumenstone Missions first to see what they had in common. NessieMo's comment from April 2018 helped with that here. (Thank you, @NessieMo !)
The followers that seem the most helpful to these missions were Vindicator Boros, Lord Maxwell Tyrosus, and Justicar Julia Celeste, all having dual abilities to defeat Legion Mission Mobs while still using Silver Hand Templars (30% success rate as opposed to Squires with 15% success rates). Lady Liadrin showed helpful with her Rebuke, and Delas Moonfang with her Consecration.
But Aponi Brightmane did not show having an appropriate special ability to specifically target the first mission "Lumenstone: Stormheim". So she is the one I deactivated for the duration of these missions in order to complete this quest (and I will /bow and drink the kool-aid by paying 250 gold to reactivate her when it is time for me to reactivate her).
It's a shame this limitation is in place, as all 6 Champions could have balanced out abilities as a team nicely throughout the duration of these missions.