Here is the full list of all 19 Slay the Princess Demo achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Finish the demo with the Spectre
Finish the demo with the Razor.
Finish the demo with the Prisoner.
Finish the demo with the Adversary.
Finish the demo with the Tower.
Finish the demo with the Nightmare.
Finish the demo with the Beast.
Finish the demo with the Damsel.
Finish the demo with the Witch
Finish the demo with the Stranger.
Why?
I don't know.
You did it! You saved everyone!
Finish a staring contest with the Princess.
I can't believe you gave her a knife.
Did your best to resist, but in the end, it didn't even matter.
Die at least 6 times across all playthroughs.
Start Chapter 2 and get the heck out of dodge.
Die.