Here is the full list of all 12 Zeitghast achievements.
Beat the Tutorial Stage.
Beat the Thunderdome.
Beat the Battle Tower.
Beat Dr. Antag.
Beat every stage on Hard Mode.
Defeat an Unkillable Monstrosity.
Collect every Collectible in the game.
Beat the Rival's challenge.
Beat the Shop Keeper's challenge.
Beat the Void Keeper's challenge.
Beat all the level challenges.
Tutorial Stage in less than 55 seconds.