Researching the Rotfiends
This is the only Witcher contract that you can find on Roche's path. In Henselt's camp, you can find this contract by the canteen on the message board. Go ahead and take it, then leave through the main exit. Before you can do anything, you have to research the Rotfiends. Learning is power, after all. You can do this by walking over to the brothel tent and looking for the book merchant outside. After figuring out why he's speaking in religious quotes, which is just a matter of patience, you can get down to business. There are a number of monster books in his inventory, but the one you need is the rotfiend one, obviously.
The other books refer to other possible contracts that you can't get on Roche's side, along with general monster information. Note also that there is no reason to buy harpy traps if you're on Roche's side of the quest, as there is no contract or real reason to kill them outright. While you're here though, it may be a good idea to buy a few books on the newer and tougher monsters. It never hurts to have a little information at your fingertips inside the game.
Once you read it, Geralt will realize that he needs to burn the bodies around the camp. If there's no easy meat for them to get, then the rotfiends will stop wandering so close to the camp. This is actually fairly easy to do. The only trick is finding the bodies. If you look over to the left side of the main road, you should be able to see a damaged cart with a few rotfiends around it. Just walk over to them and cut them down.
In order to kill the rotfiends easily, you should remember to use a silver sword. The fights actually aren't that hard, unless one manages to surprise you. Roll away from them if they start to surround you, and use some basic hit-and-run attacks to take them out. Just remember that you have to roll away or you'll get caught in their death explosion. Their final boom is probably the most deadly and risky part of the battle, as it can really hurt Geralt. Note that if you're finding the battles difficult, you can also put some necrophage oil onto your silver sword and cut through them in a quicker fashion.
The Other Bodies
After you clear the area, you can approach the three bodies and use the on screen command to set them on fire with a special version of Igni. This group of three seems to be clustered around the wagon, so it should be easy to find all of them. If you can't set the bodies on fire, then look around to see if there's another rotfiend hanging around. Since it's a context command, Geralt won't burn the bodies if he's still in combat.
The second set of bodies is just a bit past the tower near the brothel. Walk around it and to the north to find a group of soldiers waiting by the fog. You should be able to kill any rotfiends wandering around. I actually didn't see very many, and they were quite spread out. The wandering group of bandits in the north may actually be a greater threat, if you get lost. Once the area is fairly clear, you just have to hunt down the bodies. Note that they're fairly spread out, with one practically in the patrol's camp (odd that they just ignored him). The other two are in the grass near the road.
The third set is on the way to the Visionary's house. You should be able to see them fairly easily while you navigate the canyons during your trek there for the Blood Curse quest. Just keep your eyes open and look for the burned wreckage. You should be able to get the drop on the rotfiends and easily wrap up the quest.
Once all three sets of bodies have been burned, you'll be able to report your success to Proximo for some extra orens. Note that this doesn't seem to actually stop the respawning of rotfiends for now. They'll still arise in areas with burned bodies, so don't let your guard down while wandering around later in the chapter.
All information and screenshots are from Witcher 2.
If you need additional help killing the rotfiends or finding the bodies, please feel free to comment in the section below and ask for help.