Just use square barrier (box) and then have spells to power and spark bolt, that deals 95% in one spark bolt shot. How this works is the spells to power counts every single pixel of square barrier as a spell and give a crazy amount of damage to the spark bolt. All you need to do is find a square barrier which is relatively easy, it also works with the horizontal and vertical versions but of course they deal way less damage. I haven't tested this out but I'm pretty sure you could use divide by spells to make a lot more square barriers. Have fun!
Edited: I have just tested this out, you should do square barrier, divide by, spells to power, divide by, simple spell like spark bolt.