Overall, really good game! I have 3 major complaints though:
1. Your position in spell merger menu is really not clear at all (I'm using a controller btw), maybe consider adding a white border around the currently selected cell in the grid?
2. The basic spells feels waaay too weak (Even for a basic spell) when you start the game, maybe bump the damage up a tad bit?
3. The game also feels kinda rough, I don't really know how to explain it but it's like you can feel it's a demo (But that should hopefully get better as development progresses)
Otherwise, I really liked the art, music and gameplay so great job on that! I hope that you manage to put out the finished product :D