Maybe take a look at Anistuffs' playthrough to see if you can do anything differently. But it certainly is a tough level, especially if you missed the hidden blood magic spells ;)
one thing Anistuff could have optmized in general:
instead of spamming fireball or fire wall to deal 1 damage per turn, he could actually use fire ball/wall only when necessary to make people burn again and use shock instead at close range.
like, if the enemy is burning and you use shock, you make 1,5 hearts damage isntead of the 1 heart when using fire on an already burning enemy :-)
I'm stuck on the same level: the game is long enough that feeling like you can soft-lock yourself through upgrade choices or by skipping too many of the optional challenge areas is a bit frustrating. I feel like I probably could replay the game and end up with a stronger Kevin for this level, but I might just stop playing entirely instead. Because the puzzles are so stringent with how they want to be solved, it seems like set spell upgrades might create a more even experience.
Overall though, this game is extremely charming! Drawing the spells is cool, there's a ton of personality to the game and I had fun with it.