A rare, really tough mission that will require some setting up before you attempt it. Awards Abrogator Stone, required for legendary ring quest, so a mission you can't really afford to fail. In my opinion I wouldn’t attempt with less than 90% sucess rate, and even then I wasn't happy till I could hit 100%.
Requires at least 2 epic Followers, 3 are better. Each ability countered improves success by 22% with each lvl 100 follower with a counter, so if you can counter 4 of the 5 abilities you have a good (88%) chance of success.
Got 100% with
Epic Millhouse Manastorm (epic mount, reduces duration to 11 hours) ,(polymorph, counters deadly minions).
Epic Talonpriest Ishall (Dispel magic, counters Magic debuff), (mind sear, counters minion swarms).
Epic Delvar Ironfist (bone shield, counters massive strike), (dark command, counters wild aggression).
Bear in mind that secondary/tertiary follower skills are random, I had to level Delvar from rare 91 to epic, and use a re-roll abilities/traits on him ,and I had a lucky upgrade on Talonpriest Ishall. Millshouse's epic mount was random so any counter to deadly minions will do. You might be able to squeeze out a 90%+ success with 4 counters and a group/environment buff.
You'll need at least one secondary bonus to achieve 100% on this mission - for some reason, even with all 5 threats perfectly countered (and no bonuses) I was at only 99% success rate. As soon as I swapped one follower out for one with a secondary bonus, it went up to 100%. Not sure if it's a rounding issue or just strangeness with 5-threat missions, but be forewarned.
I've submitted a screenshot showing the 5-threats countered with 99% success rate.
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Missions and Followers math for maximising success chance
Warlords of Draenor Garrison Missions Guide
Warlords of Draenor Legendary Ring Guide