The Lurker Above

You must be in a raid group to enter Serpentshrine Cavern. Have a friend create a raid group with you — the second member does not need to enter the instance or remain logged in.

Travel to the Coilfang Reservoir in Zangarmarsh, dive down to the submerged entrance at 51.9, 33, and swim through the waterfall-like wall into Serpentshrine Cavern. The entrance pipe leading into the dungeon room is at 50.3, 40.8 just below the water's surface.

Before descending the deep elevator inside, prepare a Water Walking effect — an Elixir of Water Walking from the fishing dailies works well. You also need a slow fall mechanic to land gently on the water without taking fall damage and sinking: use Mage Slow Fall, Priest Levitate, a Noggenfogger Elixir until you receive the 'You Feel Light...' buff, or a Skyguard's Drape or Skywitch's Drape if you are Revered with the Sha'tari Skyguard. The two capes are on separate cooldowns, so you can buy both as a backup.

Take the elevator down — do NOT jump, as the drop is fatal without an active slow fall effect. At the bottom, cross to the smaller elevator and ride it up to the platform overlooking the raid, where Hydross the Unstable is visible to the right.

From the elevator platform, jump toward the water using your slow fall mechanic and land gently — sinking causes severe burn damage. Once on the water's surface, run diagonally to the left at roughly a 45-degree angle until you reach a circular wooden platform with a hole in the center. The Lurker Below is fished from the middle of that hole.

You need a minimum of 300 Fishing skill to fish here. Cast into the hole in the center of the wooden platform and fish up The Lurker Below to complete the achievement.