Unbroken (10 player)

"No pressure guys, it's not like we only have one try.."

After passing the gauntlet, the raid has the opportunity to repair their siege vehicles to full health before engaging the Flame Leviathan himself. This achievement forces the entire raid to ignore that option and succeed on the first try.

Minimizing damage taken during the gauntlet and maximizing iLvl of equiped gear (which increases the vehicles' health) will allow the raid to begin the fight with as much health as possible. Decreasing damage during the encounter, by not activating any Defence Systems and interrupting all Flame Vents, will further increase the life expectancy of the vehicles.

Exploit (patch 3.1):
Allow your Vehicles to be destroyed after clearing the gauntlet and retrieve your ful health respawned vehicles at the start of the instance.

I don't know if its bug or it's ment to be like that, but this achievement is really easy to get.
What we did was:
1) We cleared the gauntlet
2) Aerial units still respawn after certain time, so we let them to destroy our vechicles
3) Waited 'til our machines respawned in starting area
4) Went straight to FL and nuked him town.

As simple as that.