Lockdown!

You will need a minimum of 3 days to complete this achievement if you are lucky.
However you can still do it in a maximum of 5 days.

Suppose you have killed 0/6 of the bosses needed for the achi.
1st day - you get 2/6
2nd day - you go in and clear 6 waves. If a boss you don't need spawns: teleport out, suicide or let the mobs bring the shield to 0%. (you can suicide by drowning in the pool of water where Ichoron spawns). Without killing the first boss, you can reset the instance and try again with a chance to have another boss spawn first. After killing the first boss, you can no longer reset the instance. You can however go on and kill the second boss in the instance after wave 12, and hope you get another boss that you need for the achievement.
3rd to 5th day - repeat the above until you get all the bosses that you need.

Good luck and remember to remove your gear before the first boss spawns, in case you need to suicide.

Just a tip for those who might be getting tired of running The Violet Hold trying to get X to spawn, and getting other bosses instead. Once you clear the first 6 portals you'll get the first boss. If it's not X just logout and wait a few minutes. When you login you can run out, reset the instance, and try again. As long as you don't kill the first boss you can repeatedly reset the instance until you get X as the first boss.

Taken from the Dehydration achievement comment section. God knows why this comment section is so useless and vague.

This achievement, like every other random achievement, sucks. You can be extremely lucky or you need to wait for one boss for weeks. I did this instance every f!cking day and we're waiting for Zuramat for weeks.
Im not sure if every boss has the same chance to appear, other opinions could be helpful here.