All Bark, All Bite

The achievement itself isn't bugged people have just misunderstood how it works. The boss gains a shield when he starts to eat someone or some people [spell=378022]. To meet the criteria and thus do the requirements of the achievement all you have to do is have all 5 people get eaten AND NOT hit the shield while he is still eating, we just had everyone afk while he was eating and just focus on staying alive, after he's finished he'll just spit you all out and you get a debuff [spell=383875], this hurts so it is advised you use some defensives while you still have it, i would suggest moving pretty quickly after he spits you out as well he'll do some breath like mechanic + spawn adds and this will pretty much be a one shot. After we clarified we got the achievement we just popped heroism and nuked the boss + adds and we all got it! Hope this helps you also get the achievement. Have fun! ^.^

Pretty easy to solo:

1. 
   
2. Just stand in his first consume attack that he uses when his energy bar hits 100.
3. 
   
4. Get consumed.
5. 
   
6. Let the consume cast complete.
7. 
   
8. Once you pop back out, kill the boss.
9. 
   

Just be sure to not do it on a pet class or a class that spawns any objects as it will reset the boss.

No need to be a group of 5 to do it. Did it solo with my paladin tank. I was eaten, waited until the boss spit me by healing myself + cd just in case (didn't use divine shield just to be sure). Then I killed it.