Replay a chapter in Magicka

Letting Jormungandr run into barriers seems to be an effective tactic. I used stationary shields (E + right click) and stone walls (E + D + right click) and just let the worm run into them. They do pretty decent damage all by themselves. With E + 4*D you get even stronger stone walls, those work particularly well.

Just running around and covering the whole area with stone walls ensures that Jormungandr will hit something when he surfaces.

According to this video

showing how to get it, it is only usable in multiplayer mode, as you can't launch yourself with mines but only other players (if you have a second controller it is doable on a single computer though)