I find very interesting in battle this mix between strategy (what spell to do on who) and dexterity (execute the spell). Even if your strategy is good, and your character is strong, if you aren't skilled enough to execute the spell it won't work.
I didn't play until the end so maybe my comment won't be interesting. I guess it become more and more complex in strategy, but I think the spell casting can quickly become boring (always making exactly the same pattern for a given spell). Maybe it would be good to make a set of pattern for each spell, and one of them is randomly chosen at each cast ?
Also, I didn't quite understand the use of the runes. For me it was just a shortest way to cast spell, but in the end I always used the book because I was too lazy to memorize them. Maybe it could be a good idea to make the spell more effective if the player uses the runes and not the book ? That would push players to memorize spell and use the runes, plus it's very stylish to use the runes haha :p.
Keep creating games !

Thank you very much ! Yes the platformer has always been a problem... There is another way to solve it, but when people don't find it, it becomes frustrating for the most part.

Thank you very much for your great feedback !
I'm very happy that you had fun with the game;
I planning to make something greater on the same concept ! :)