Hey, thanks for the feedback and sorry the slime boss is giving you trouble! There are a few playthroughs on YouTube that might give you some tricks to use, there's some stuff in the game that isn't tutorialized well yet.
As for the difficulty itself, we're definitely planning on making this first area a little easier and moving the difficulty into later areas and higher difficulty levels (like ascensions in Slay the Spire and Monster Train) - we just kept it a bit harder while we work on more content so that there was some room for replayability and trying out the different orbs and relics.
Thanks for playing and I hope you'll give the demo another try when it gets closer to 1.0!