You can get this quest by stumbling across the group of guards in Lobinden. They're threatening a young elf woman, Malena, close to the center of the village. If you approach the group, you'll have the opportunity to investigate. She'll proclaim her innocence, while the guards will blame her for the deaths of two men in an ambush. This sounds like a fairly standard quest setup for us.
You'll need to take a bit of a long haul to the far left of Flotsam. They'll automatically go to a new spot by the cave and wait for you. I wouldn't suggest just running over to start the quest though, because you really want to prepare for this. This is one case where you need some traps and lots of bombs. You can get by with just a few bombs, if you want. The traps are vital though. There are too many nekkers inside. They will surround you and then break you down, if you don't use traps to split them up and pick them off around the edges. Snare traps can be enough, but larger traps are obviously better if you can spare the materials or orens for them.
Also make sure that you take a Swallow potion before you go inside. That extra bit of healing is always nice.
When you're ready, just walk along the road to the far side and drop down off of a ledge to get to the lower level by the beach. Move around the path to reach the group. Move forward to the cave and go inside.
The Nekker Cave
For now, we're just following the very long blood trail. If you're having trouble seeing, a Cat potion might help. I personally don't like the view effects, but it can certainly help you scout the cave well.
Most of the nekkers are right by the front. The good news is that once you beat this group, you're basically done. There should just be a few groups of stragglers deeper inside after the fight. This first engagement will require some tricks though. The one important note is that if you choose to run back toward the entrance, they will stop chasing you, for the most part. If you need a break, falling back is an option.
Once you find them, quickly set some traps near the bottleneck in the tunnel, and fall back. Let them hit your traps and take some damage, then jump in with a Quen on and pick a few off. Then just fall back, let them cool off, and rinse and repeat the trick. If you want, you can go in and just chuck a few more bombs around. It's really your call though.
Once you've weakened the main group enough to take them out, you can move forward and track the blood trail to the big spot before the ledge. Jump up, and follow it down the long path. Climb the ledge again to find the two bodies riddled with Scoia-tael arrows, and a locked trapdoor above them. Examine them to confirm the find, and then backtrack out. You can take a shortcut by dropping off of the ledge just to the right of the bodies.
You have a bit of an option here. You can tell them that she's definitely guilty, in which case she'll just hang, or you can choose to listen to her one more time and follow her to see her evidence. It's just another ambush though. At the end of the walk, you and the guards will face five Scoia'tael. Two in the back and three in the front.
The good news is that there are plenty of targets for them to fight. Just move in toward the front, use a parry and then strike out at them. If you have the riposte skill, then you'll finish this in a few seconds. If you can keep a Quen up, you can be as aggressive as you want. Just keep up the pressure and pick them off as they fight the guards.
Apparently Malena can vary a bit at this point. She may run away during the fight, which will make you have to chase her down in some ruins nearby. You then get a final decision to spare or kill her. If she doesn't run though, the guards will arrest her immediately.
The quest will wrap up at this point. Loot what you want from the bodies, and then leave.
All information and screenshots from Witcher 2
Due to the branching nature of the game, my experience may differ from your game. Please feel free to note any differences that you find.