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In the Shaman's Path, after the Stalkers part and a couple of swinging bridges, there's a rock arch and above it, on the right, I can spot a crate with my focus but I can't figure out a way to reach it.
Since it may contain something rare or unique I don't wanna miss it.
As dg99 pointed out, and from my own experience as well, there does not seem to be a limit to how many enemies you can tag at once. It would seem that it is only limited by the render distance of the game; sometimes human enemies glow a faint orange to signify their distance away, and occasionally I cannot tag them. Otherwise, you can tag all enemies that are within range of your Focus.
My favorite strategy that blends stealth with action is to create a mess of tripwires between you and your prey while utilizing Concentration and Tearblast arrows to stealthily take out opponents from afar.
From what I can tell, setting tripwires (though very loud) is perceived as silent by Machines. By that I mean you can place them literally on top of/through Machines and they won't have any idea who did it. Even when walking into tripwires, Machines typically will not find you if you're hiding behind rocks or in tall grass.
Once you've effectively set up a no-man's-land of crisscrossing tripwires in every direction (bear in mind that the maximum placeable at any time is 16), you can try getting headshots/destroying components of Machines. Another way to set these up is to basically fence a group of Machines by using tripwires. Electric will immobilize them and allow you to get a Critical Hit on however many were affected, while Blast wires will cause massive damage (and likely remove components).
If you are able to use them (they are on the Sharpshot bows), Tearblast arrows will do little to no damage, but will quickly remove the components of Machines (which effectively incapacitates them) and allow you to take them down more easily. Concentration will also help you hit these components and weak spots by slowing down time and zooming in, so it's very helpful for this.
If the Machine you're hunting notices you, they will most likely charge. At this point, they're extremely likely to run into at least one wire of your giant tripwire trap and then begin the process of stumbling into every other tripwire you've set. If this hasn't killed your prey yet, it should have come very close to doing so, at which point, you can use your weapon of choice to finish them off.
Best Answer
For the crate with a zip wire leading down from it near the waterfall operated swing bridge:
Drop down in to the stream below the bridge and move downstream until you see yellow climbing points leading up to the ledge with the crate. Then just climb up.