Restoring Order

Restoring Order is a quest and achievement/trophy in the Fallout 4 add-on Automatron.

Detailed walkthrough
The Sole Survivor is tasked with finding and stopping the Mechanist once and for all, so head to the RobCo sales & service center where The Mechanist's lair is hidden.

Once inside, player character must use their robot, with an attached M-SAT, on several devices stuck to walls, to open the blast proof doors throughout the lair. After a lot of battling, the player character will finally make it into the Mechanist's war room. Inside, they will battle several waves of increasingly tough robots, the last one being a Duelbot.

Alternatively, if the player has collected all three supervisor holotapes and before going into the war room, the player can avoid the final battle with the waves of robots. If the player character goes all the way back to near the beginning of the lair, where the first set of robots were fought, there is a decontamination corridor with a security room overlooking it. This room can be accessed either by a locked door on one side or a hole in the wall on the other. In this room is an elevator that is initially not working. Next to it on the wall is a master-level locked "elevator access terminal" that can manually override the lockdown. After unlocking the terminal, then inserting each supervisor holotape and playing them, the three voice authorizations will override the lockdown and the elevator will work. At this point the player can go up to the control room, surprise the Mechanist, and a speech check will allow the player to make the Mechanist stand down.

Once it has been defeated, the power will fail and a speech check will allow the player to make the Mechanist stand down. Note that the dialog here is timed, so if you wait too long to speak with the Mechanist, she will turn hostile. After explaining herself, she will remove her helmet, revealing herself to be Isabel Cruz.

She will then thank the Sole Survivor, before giving them her costume and walking away. She will usually also give the side quest Rogue Robot. Ada will also thank the player character, at which point the main storyline of Automatron is complete.

Bugs

 * After the power fails, the Mechanist may not speak with you, making the quest impossible to complete. This may be due to shooting any of the robots on the assembly line after the first wave of the final battle.
 * Loading a save from before the start of the battle may fix the problem.
 * Exiting the game entirely when the Mechanist won't speak with you and then starting the game and loading the same save may also fix the problem (it did for me on xbox one).
 * Wearing power armor with a Targeting HUD mod may make the Mechanist & Sparks become hostile towards you. Removing the mod or the entire helmet or exiting your power armor can fix this..
 * When reaching the final battle with the Mechanist, it is possible to access the workbench to the right of their control room and place turrets between waves of enemies if the player character has sufficient crafting materials in their inventory.