Hertz Locker

We did this 2 days ago, and you don't really get anything except the achievement if you don't have rank 3.

Edit:

After you learned your rank 3 essence and you have the achievement you can talk to Prince Erazmin in Rustbolt to get your rank 4 essence for free.

Awarded when completing Mechagon on Hard mode, without anyone dying. Alongside Rank 4 of [spell=299368] via Prince Erazmin.

Activating Hard mode:

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• The [npc=150190] hovers over one of the first three bosses when you enter the dungeon. Normally, you would head to whatever boss doesn't have the HK-8 Aerial Oppression Unit over it, but for hard mode you must defeat each of the first three bosses while HK-8 Aerial Oppression Unit is over it!
  This causes you to have to deal with Walkie Shockie X1 robots during the fight, which are immune to damage (but not crowd control) and slowly follow random players while using Static Discharge. Upon reaching full energy they will self destruct.
  
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• Upon defeating the first three bosses while [npc=150190] is above them, you will head to fight the aerial unit. This fight will replace the normal [npc=150295] with a [npc=155760], a harder hitting mob that also uses a move called Fulminating Burst, which adds a stacking mechanic throughout the fight.
  The Walkie Shockie X1 robots you dealt with in the first three bosses return in this fight as Walkie Shockie X2 robots, which function the same but also have beams that connect between them which hurt when walked through.
  The final change is reduced time to complete the hack in phase 2.
  
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• After defeating [npc=150190] you can continue the dungeon until you get to K.U.-J.0.
  There will be a clickable pile of robo-waste on the ground that you must click before engaging the boss.
  The only change to this fight is the addition of [spell=301850], which adds a stacking slow.
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• The final part of hard mode involves the fight with King Mechagon.
  There will be a small red button on a pedestal that must be clicked before the encounter.
  Clicking the button will add a robot to the fight that does nothing but deplete its own energy bar over the course of the fight from 100 to 0. Upon reaching 0 energy, the party will instantly wipe.
  To counter this, there are 4 panels around the room that will activate when the robot reaches 35 energy, roughly every 45 seconds.
  These panels will all display a color and icon in a random order before flashing a red skull and activating an input sequence just before the final 10 energy countdown.
  4 members of the group must each stand at one of these panels just before the input sequence and click them in the correct order that they lit up in to bring the robot's energy back to 100 and start the process over until the boss is defeated.
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Unfortunately, this achievement becomes unobtainable with the Shadowlands pre-patch.

I can confirm that you can try this multiples times within the same week. If any of the party members dies you can still clear the dungeon, zone out, create a new lock out using an alt, zone in all of the other party members, then log in with your regular character and try again.

PD: Dying on trash still fails this achievement.