Well, as I am running out of options, I think is valid to ask for help here...
The achievement is becoming harder day after day with less and less people doing Garrison content.
I've been playing since the start of WoD and never got a single item, and I only need the Iron Horde boss. Missed him in the last day of WoD for a few moments and I couldn't feel any worse.
you should have used the addon "premade group finder" it scans lfg for keywords, like the names of the bosses or "garrison boss". Still worth it to get it in case somone gets the boss youre looking for