Survive for 90 seconds on the pillars in the Swollen Vault.
This achievement requires the Isle of Thunder to be in Stage 5 (or a way to access the Stage 5 area).
The achievement itself is quite simple, honestly. The most important thing to know is that the pattern is fixed but changes daily, so if you watch the tiles for 30 seconds to a minute, you can jump in and just follow the pattern.
Got this last night with Aliok and a couple guildies, though we're still on Stage 1. Nothing exploitative, just kite a Bluewater Shark over to Stormsea Landing where the dock is and let the shark kill you. Corpse run back and resurrect on the dock itself. The door to the Swollen Vault is nearby and unlocked, so go inside and you'll see the 5x5 grid of platforms being guarded by a mob (whose only special ability is Decapitate AKA damage & knockback).
There is a post by banana77 over at Wowhead which gives good directions on the patterns used by the... thingie.