This achievement is the "worse" ending to the game and should be considered
missable.
Spoiler free versionAfter the boss in the
Underground Sorcery Lab you are given a shard that allows you to teleport short distances. Use this shard to access the
Hall of Termination and defeat the boss here.
*** Spoiler - click to reveal ***After you defeat Bathin in the Underground Sorcery Lab (after the train scene), he will give you the Reflector Ray shard that lets you teleport short distances and reflect off panels. Travel to Livre Ex Machina and use this ability to reach the Hall of Termination. Once here, kill Gebel as you normally would in any old boss fight.
This must be done before you progress to the
Oriental Sorcery Lab and defeat the boss there. If you do, you will earn the
Usurper achievement instead.
You will get a few lines of dialogue followed by a Game Over screen, but not to worry, the game will revert to your last save point.