Only the Penitent...

Everything you need to know about this achievement.

1. 
   
2. You need 2 people (1 on each side)
3. 
   
4. You can use /sit /kneel /lie /sleep or just press your sit button (Default is X)
5. 
   
6. You must not be shapeshifted
7. 
   
8. You must not be in bear/cat/wolf form
9. 
   
10. You must not have any companions/pets/minions/elementals out
11. 
    
12. You must not have any gear equipped that causes anything to spawn (eg. [item=77191])
13. 
    
14. It takes a few seconds to activate the flames so I suggest you call out on voice comms when to do it, wait until a cast is finished then click the orbs at the same time.
15. 
    
16. If one person gets hit it fails the achieve.
17. 
    
18. It is required for Glory of the Firelands Raider
19. 
    
20. Failing this achievement awards -50 dkp.
21. 
    

Good luck.

As of patch 8.2.5 this no longer requires 6 people. I did today with a friend of mine and i got the achievment.

Just did this with a friend!

YOU CAN DO IT WITH 2 PEOPLE!
Same tactic, it dropped the meta achievement with only 1 person on each orb!
Don't know when this was changed, but please push this comment on top so others don't lose hope :)

Cheers and birb on!