Wailing Caverns

Here's a tip for people looking to do this achievement on high level character. When waves of mobs start spawning wait for them to leave water (or attack Disciple) before you kill them. Otherwise event may become bugged and you will have to reset and do whole instance again.

To get the event started you only have to kill the 4 <Fanglord> bosses. So if you are a high level just entering for the achievement then there is no need to kill all of the bosses, the only ones needed to be killed are:

Lady Anacondra
Lord Cobrahn
Lord Pythas
Lord Serpentis

Once these four bosses are dead head back to Disciple of Naralex standing at the entrance and start the escort the summon Mutanus the Devourer.

After 2 waves of mobs the boss will appear, kill him and Achievement unlocked!

Patch 3 live.

To complete this achievement you need to clear all the bosses.

Once you have done that you then need to talk to the Tauren Druid at the enterence of the instance and start the event.

More Info

Its an escort so you will need to kill 3 level 20 non elite mobs on the way.

After he arrives at the end of the escort, 3 waves spawn with a small break in the middle of each. First 2 waves are non elite level 20 mobs. Then the last guy is will reward the Achievement.

Escort takes about 10 min to complete.