Sleep with the Fishes

At the entrance of Mug'Zee's room. there is a teleport pad next to the door to the left when entering boss's room. The pad will teleport you to the aquarium that you ran past earlier.

There is an NPC murloc named Fish Stix got grounded to the bottom of aquarium by a cement block. Destroy the block to free the murloc and he will swim up the pad to join boss's room with the raid. If the raid is wiped or reset for any reason, you will have to free him again. Talk to him before pulling boss.

During boss encounter, Fish Stix will periodically summon Sleepy Fish into boss arena, enclosed by 1hp bubble, you need to rotate raid members to run to Sleepy Fish, pop the bubble, once the fish is on the ground, target it and do emote /sleep.
Doing so give you Slept with the Fishes debuff that let you know you met the criteria for each raid member. The distance for it to work is in melee range.

Achievement is granted when boss is down with everyone have Slept with the Fishes debuff.

I would suggest making a macro like this:
/target Sleepy Fish
/ping [@target]
/sleep

Additionally you need to be basically standing on top of the fish, and while multiple people can /sleep the same fish, the window is incredibly tight as the fish despawns shortly after.

Try to finish the achievement before the mechanics get too complicated.

Step by step guide for all the achievements in Glory of the Liberation of Undermine Raider: https://youtu.be/LotBGmWtn-o

At the entrance of Mug’Zee’s lair, there’s a teleporter that takes players down into an aquarium like zone, swim down and you’ll see [npc=236828] being held down by a cement block. Destroy the cement block to free Fish Stix. There’s no need for everyone to go down, just enough players to break the cement block quickly.

After freeing the Fish Stix, swim back up and start the fight. During the fight you will notice the Fish Stix throwing fish onto the floor area. Each player needs to go up to a fish and type /sleep. Doing this will cause the fish to despawn so each player has to wait for their own fish. All the players need to do this for the achievement to count. You’ll also notice fish flying around in bubbles, you can DPS the bubble to drop the fish. Be sure you’re close enough to the fish when you do the sleep action because if you’re a bit far away it may not count.

Keep track using the achievement tracker and see if your name pops up when doing it. When all the players have slept with a fishy, continue the boss fight and you’ll earn the achievement when the boss is defeated.