How do you beat this guy I'm having trouble.

I guess attrition did it.

I threw big rocks that I set on fire into his mouth when he bow's down to try and bite you when you are close to him. It took quite awhile to take him down like this :)

I hid behind the tree and shot lightning that took a while.

Yeah I also chose fire. But I used shield and arcane with it while holding shift to get the circle of mines. Stand still until he's about to come up from underneath you then plant it. You'll be knocked out of the way and survive if your health was full. Refill health and avoid his attacks while he burns until he goes back under ground. Repeat until he's dead.

I laid grease down in front of him once he came up to attack, then lit it on fire.  When he goes to attack you he ignites, super easy win.

I just laid down shields in his path. He smacks his head on 'em and goes down fairly quickly.

That's the snake right?
 
Nothing was doing it until I gave a strong earth attack a shot. I can't remember what I combined it with, likely fire, but whenever he goes to strike you launch it at him and he'll get knocked back. Once I figured that out he never touched me the rest of the fight.
 
Did a decent amount of damage too.

I used stone walls combined with ice to make it more interesting. 
 
Actually I also used meteor shower on my first try and haste to run around which was a lot more fun, but the meteors didn't hit him that often. :)

Combine Water and Ice five times, and cast it AoE style. It shoots up huge icicles and it should hit the Jormungandr even if he's not bending down.

Absolute easiest way is to simply lay area shields right before he comes up. Just stand your ground - when he tries to come up underneath you move over just so he doesn't pop beneath you and then Shift + E and he'll smack against the shield. When he starts pouncing out the ground in swimming dash, just stand your ground and do the same - Area Shield. 
 
May not be the quickest, but it's certainly makes that "Boss Battle" a joke. When you run through it a second time - cast Time Warp when he pops out his head and then just fry him 2-3 times with Lightning Bolt. 

@Briguile: 
 Grease is found in chapter1 when you first get out of the main building, you get to a lake & hear something close to a Wilhelm scream, use frost & earth to freeze parts of the water faster than the frost spray (note I found something similar to this in the next chapter or two where you freeze a river to get to a hidden spot for some possible gear).  
 
Easiest way I found with this thing as most bosses are rather tedious while the rest of the game can be fun at times, is shield + earth +frost (or fire and/or arcane). When the serpent either goes underground or charges you, just drop the combo with the shift bar down. If you just RMB this it will just create an arc that might miss, easier to just shift to get a circle around you. The serpent will either charge into the rocks or burrow up into them doing decent amounts of damage without having to worry about aiming or charging up a spell. Aside from this the only spell you really need to do is heal to offset the poison (or use life & shield RMB for healing mines to run over).

Dude just bring up rock shields everywhere, he'll be dead in like 15 seconds.

I just used Ice (Water + Cold) + Ice + Ice + Ice + Earth. Stand right outside his range and charge it up, so that when he lunges at you you can hit him with the projectile. He'll be dead in one to two shots.

Looks Like I might need to restart to get grease new topic what to do about those fucking jumpers do you guys shield up? Use Bolts or What? Also Ramming guys in Chapter 3

I just made a fire-rock-shield when he had his head down closer to the ground; right before he would attack. It takes a long time this way, but it's fairly easy. 

I just did lightning and fire got close and shocked his face. Playthrough 2, thunderbolt.