'Twas brillig, and the slithy toves did gyre and gimble in the wabe

The way I found that works is Din's Fire. Back him into a corner (which there shouldn't be, but we'll ignore the practicalities of invisible walls), give him a dose of Din's Fire, and repeat. With the magic upgrade you can get all but two of the hits needed on him, and it's not too hard to hit him twice with the sword.