Get hc achievements from SoO bosses

So i was in a lfr with a friend and i had to leave mid fight. When they defeated the boss, i was in stormwind and i received the heroic:iron juggernaut achievement. Prior to that I was invited in the main raid while it was set on heroic.

So from what i assume: queue with a person in lfr, leave midfight. Ask someone to invite you and make a raid and set to heroic and ask him to go inside, when the boss from lfr will die you will get the achievement.