Every time I try to do these, I end up sitting in the group finder for over an hour missing just a tank or healer, and it's a constant hamster wheel of people leaving. It's a shame they didn't give these achievements an incentive to do. I can solo all but the last two bosses on mythic, but not the achievements.
If anyone wants to join me or give me magical tips for soloing them, please post here. I'm on US-Alliance. We might be able to go 4-5 DPS and avoid the wait for a healer if everyone is super geared. (Try selling that one to people in the group finder!)