How do I defeat Jormungandr, the snake/worm boss at the end of Chapter 2?

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.

I used the 5 charge stone method. Basically, stand near him when he pops out and charge up with the mouse cursor right where he is coming out of the ground. As soon as he lowers his head to strike you, release it. It should do a couple thousand damage.

One very easy way is to use the Grease Magick (located in chapter one on the eastern side of the large river just outside the starting area). When you see the boss coming out of the ground, just drop some Grease and set it on fire. Then run around to lure him into striking you so that he'll put his head into the flaming grease when he does. Being covered in burning grease doesn't make him retreat into the ground, so it's possible to kill him in a few seconds using this method (though more likely he'll duck into the ground first).