|Defeat in Azjol-Nerub on Heroic Difficulty before he webs the top doors and prevents more creatures from spawning.|
To complete this achievement the party must reach Hadronox before he ascends the tunnel. This can either be achieved by jumping off the bridge or if a hunter/rogue/night elf is in the group the hunter/rogue can pull the Crushers that usually initiate the fight and feign death/vanish/shadowmeld. The use of feign death/vanish/shadowmeld will cause the crushers to De-spawn and the party can simply walk down the tunnel to confront Hadronox as the fight re-sets.
It is important to avoid the mobs that constantly run down the tunnel to fight Hadronox, however they do not continuously spawn so it is possible to reach Hadronox without aggroing them. When you pull Hadronox it is imperative not to aggro the mobs in anyway, thus any form of AOE should be avoided. The fight is very simple and in fact easier than the regular fight since the mobs help kill Hadronox.
If any member of the group does accidentally aggro the mobs then it it effectively means a wipe, not only will they attack you rather than Hadronox, when they die they will heal Hadronox.