So apparently you can use spells during the shape fights, at first I thought that it was because I was in debug mode but it works in normal gameplay too, the blue bar to the left is your "mana" at it has 4 leves, if you press left shift it cycles through the spells and after a couple of second they are cast, each one requiring from 1 to 4 notches of mana, the fist one heals you (I think), the second one gives you a random power up that shoots enemies, the third gives you a trinket after the fight (gives you +0.1% exp multiplier which sounds pretty useless 10% more exp) and the fourth one a stronger power up (I think).
Edit: It gives you a +0.1 exp multiplier, so 10% more xp, not 0.1%, it's actually really useful, I'm just dumb.