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.