Kill legendary captains from the marauder tribe to get a hold of pieces of the marauder gear set, then while you have these pieces equipped almost every orc in the game drops mirian when they die. Doing online conquests with marauder gear is an excellent way to farm up mirian in bulk.
I would also say that destroying gear works wonders. I am post game and I have so much gear, it's overwhelming. I recently decided to start destroying gear and I ended up with tons of Mirian. I still have a lot more to destroy!