Here is the full list of all 12 Poltergeist: A Pixelated Horror achievements.
Finish all the Eighties Era levels
Beat all Larry Zucchini's levels
Finish all the Classic Era levels
Beat your first level in the game.
Beat all Nicole K.'s levels
Beat all Reverend Morris' levels
Beat all of The Boss' levels
Unlock the Office Era levels
Finish all the Modern Era levels
Finish all the Office Era levels
Activate the secret glitched objects
Finish the time trial mode in less than 90 minutes