Yeeep, I can feel your pain...
The boss appears at the end of level 19. He is quite tough. If you haven't invested hearts into upgrading skills it will be very difficult to defeat him. Boosting your defense against enemies might save you. Focus on defending against flies and the troll and click on the boss when he opens his mouth.
The old demo was quite repetitive and difficult. Now we changed the gameplay to more varied and shorter. The new demo will be released this spring ;)