If you want to be extra safe, then you can choose to get this Ostmurk for your Mongoose potion. It is nice, but not completely necessary. The ideal is that you never get hit, but, it also doesn't make much sense to turn down a quest. There's extra loot in the cave anyway, so I figure that it's worth it.
Move forward and watch out for nekkers and endregas along the path. If you don't plan to kill the nests, then just keep moving and only engage the ones that chase you. Also note that none of the monsters out here like one another. It's fairly easy to just let them kill one another.
Nekkers will die to a few quick silver sword strikes. If you keep jumping between them, they shouldn't even land a hit. Warriors are slightly worse, but not horrible. Just use Quen if you see the medallion go red, and keep moving. Save often too!
The cave is a marked cave behind a waterfall at the back. There are a few drowners in the water, so be ready for that. Again, it won't be horrible, if you have Quen and you can move quickly between them. The pond lets you move around a lot, and you should be able to pick them off fairly easily (in comparison to other fights in the forest at least).
Move through the waterfall to enter the cave and start your search. After a few turns you'll run into some nekkers. The spawns seem to be semi-random. I've seen warriors in different parts, so I can't get too specific. Note that the normal ones should die in about three fast strikes. If you're locking onto them, you should also be leaping around so much that you barely get hit. Warriors are annoying, but you can leave them for last. Just take out the weak ones and then back the warrior into a wall. In one-on-one combat, you should be able to just beat them to death without any dodging.
The only real trick to the Ostmurk cave, is noting that there is one hotspot in the middle, with two spawning points. Don't just keep moving at this point. When you encounter the nekker spawns, right after the first two, fall back a bit and fight them on your terms. If you try to push forward, they'll easily overwhelm you from all sides. Keeping your back to the fairly cleared side is much better. Then just work through the weak ones as they rush you and grind down the warriors. It is possible for them to spawn behind you, but there shouldn't be more than a pair.
Once the tide is stemmed, loot the bodies and look for the dead man in the side cave with some valuable trader junk on him. Then just turn the corner and push forward against the occasional nekker until you find the glowing Ostmurk on the side of the cave. There's one final foe, a greater rotfiend, here. There's just one though, so you should be able to just hit it repeatedly and end the battle.. Grab the Ostmurk, and then just backtrack out.
Do be careful though, since it looks like the pool outside is a spawning point for some Scoia'tel. You may have to handle them on your way out.
Making the potion isn't hard. Just meditate and mix up some Mongoose potion with any of the herbs you can pick up around the forest. Once you have it, you'll just need to hold onto it until the big fight.