Nearly Indestructible

Soloable in BFA. Simple steps:

1. 
   
2. Clear Trash
3. 
   
4. Activate Boss
5. 
   
6. Stand in between the two cannons
7. 
   
8. Kill EVERYTHING as soon as it appears, especially siege vehicles
9. 
   
10. Ignore all Felfire Munitions
11. 
    
12. Eventually enemies will stop spawning and you can go round picking up the munitions and taking them to the cannons. Which should still be at 100% HP
13. 
    
14. Get Achievement
15. 
    

Pre 6.2 Notes:

-Focus Fire Felfire Crushers and Artillery.
-Cannons can take at most 9.9% of damage. So keep an eye on cannon health.
-Felfire Artillery should be immediately be jumped on as soon as it spawns.

Will edit with release.

Having completed the meta recently. Just wanted to give a quick run down of this one.

Achieve goal, complete Hellfire Assault encounter while keeping BOTH cannons above 90% hp.

Doing this on normal witth decent dps isn't too difficult. As soon as a Siege Vehicle is in range, it MUST be focused down to prevent it from causing damage to the two large cannons.

Felfire Artillery, attempt to hit the Hellfire Cannons with their primary attack
Felfire Flamebelcher, will do a cone attack in the direction of the Cannons
Felfire Crusher, does a lot of damage every 0.5 seconds in a short ranged front cone goes directly for a cannon.
Felfire Demolisher (heroic+), Will fire mortars at a targeted location. Mostly a raid hazard.

To complete this achieve, the kill order should be Crusher>Artillery>Flamebelcher>Demolisher. Also try to becareful to not push multiple Felcasters below 50% at once, since that increases the amount of damage the raid will take and make this a bit more difficult.

It is by far one of the easiest achievements in the meta, just make killing the Siege vehicles your top priority.