Ah, De Rol Le. For this boss, you'll find yourself on a raft... in a sewer... racing downstream. How lovely. And guess what? The boss is a giant worm. Once again, lovely. Oh well, all the better to kill it right? But of course. =p After the intro, De Rol Le will use several attacks. I'll list them in the order I observed while fighting him in Very Hard mode, his attack patterns may be different in Normal/Hard, but I doubt it. First off, he'll wave his tail at you and scatter spikes all over the raft. (5 of them, to be precise) Kill the topmost left spike and hide in the corner, as all the other spikes will blow up and cause damage if you don't get out of the way. Once he's done that, he'll wave up and down at the side of the raft, shooting purple orbs at you in two volleys.
Once he gets that out of his system, he'll jump up onto the raft. This is the chance for hunters to get in their close range damage, all the rest of the time you'll be forced to use guns or magic on him. I like to attack his head and knock off his mask, as when you do you get to attack his softer head underneath and it spews green blood all over and he squeals, but hey, that's just me. =p No, I don't torture kittens in my basement, you pervert. I happen to love cats, and this is just a game. Got that? Good. =p Now that that's clear, you will probably either want to use a multi hit weapon (like a Dagger) to hit him repeatedly, or a multi enemy weapon (like a Sword) to hit his many body parts simultaneously. Don't get too comfy though, as he sits on the raft he'll stab stab you with his tentacles, so move around a bit to get out of the way. He'll only target one person per each time he lands on the raft, so that person should run around to keep the others from taking damage. (If you stand near everyone and he hits you, they'll take damage as well.)
Now he'll send off some more orbs, and jump clear over the raft. He's rather maneuverable for a giant worm, eh? Now he'll launch spikes again, but this time only three of them, and they'll gather around a single person. This player should run away from the other players, so when the spikes explode the team doesn't all take damage. You can try to kill the spikes though, and if you have enough power in your team you may succeed before they blow. Now he'll launch orbs yet again and jump onto the raft. Beat him hard now, because things are about to get a bit sticky. After he leaps off, he somehow magically flies through the air (OK, now he's really maneuverable) and drops rocks all over the raft. Some players like to hide in the upper right corner (when you're facing him) but I don't think it always offers protection from being smashed. Now he lands back in the sludge and the lights go out. Ack! It's not too dark though, so you can still see. He'll swim near the raft, and rise up out of the water. At this point run to the side of the raft opposite him and get in the center. If you do this, his next attack won't hit you. If you don't you'll have to run around trying to dodge it. He'll now fire a purple laser every few seconds, sweeping over the raft. If you don't hide, prepare to take heavy damage. If you have a force and a ranger, the ranger can stand in the middle of the raft and shoot De Rol Le's head, while the force heals him after every blast. If you're not that brave, just hide until he stops.
Now he'll use the homing spikes again, same drill as before. Then he uses the laser again. My, isn't he repetitive? I suppose whilst sitting in a sewer he has lots of time to think about how to kill people, but come on... Finally the lights come back on, and he'll start using his other attacks again. They didn't seem to be in order the second time around when I was noting all of this down, (I wanted to make sure I got the order right) so just reuse the above information and you should come out victorious. Note that he can drop a rare armour, Parasite Wear: De Rol, so be on the lookout for that. Have fun. =p