Helheim Hath No Fury

There is a Val'kyr next to 2nd boss and she is losing HP all the time, main goal is to kill the boss before Val'kyr dies.
Once you are inside the jail in the ship. There will be an emote on the chat saying: "Harbaron begins to drain the soul of the Captured Val'kyr."

This means you have 3min 30sec time to get to the 2nd boss AND kill him within the time limit.

For proper high geared group this is no problem to deal with.

For low item lvl players simple guide to do this:

1. 
   
2. Everyone gets Draenic Invisibility Potion (18sec invisibility)
3. 
   
4. Once inside the ship use /stopwatch 0:3:30 and activate it. Pull first 2 monsters in front of you and 2 monsters on the left side
5. 
   
6. Watch out patrolling monster, wait until he walks back to the stairs (He can see invisible players)
7. 
   
8. Use the potion and run past all the monster until the last monster sees you, pull it upstairs and kill him and last 2 monsters just before 2nd boss room.
9. 
   
10. Pull the boss and pull out max DPS.
11. 
    
12. Kill the boss before time runs out and enjoy the achievement.
13. 
    

Chat will notify emote if you fail during encounter that Val'kyrs soul belongs to Helheim and she will also disappear from that room.

All you have to do is get to the 2nd boss fast once you kill the first boss. That's it.

For those wondering who never did this older content: you have to click on the horn behind the first boss to proceed to the next part of the dungeon. This is still doable and easily soloable, just kill the first two bosses fast and I had no issue.