Strelok's damn good at fighting off powerful, hard to kill, and stealthy enemies with very strong attacks: bloodsuckers (and poltergeists, but they're not as damaging). Is Tidus invisible? No. Does he stand out against the jungle? Yes, yes he does. A sword, a shield, and multiple chains are going to be pretty loud when Tidus runs around in the jungle.
Also, while Strelok is not god-like, he is a man who goes into a radioactive hellhole full of deadly anomalies and extremely dangerous mutants (bloodsuckers, controllers, pseudogiants, poltergeists, even those swarms of wild dogs), where almost everyone is out to kill him (in an area where practically everyone is a badass), and not just survives, but thrives. One man, constantly outnumbered and outgunned, and he becomes the highest-ranked STALKER in the zone (Try winning the game as intended without this happening. Just try.). Heck, just to win the final battle of SoC, he pretty much needs to be a hyper-accurate gunman.
Has he killed extremely powerful melee combatants? Yes. Can he survive their blows for a while? Yes.
Has he killed swarms of invisible attackers? Yes.
Has he killed fast-moving melee specialists? Yes.
Has he killed enemies which mess with his aim from a long range significantly, slow him down, stagger his movement, make his vision extremely blurry, make him sway around while he tries to fire, and repeatedly try to blow his brain apart, while being able to take absurd amounts of damage? YES.
Remember, the Zone's mutants aren't groaning, shambling messes (with the exception of Fleshes). They're things like the fast-moving swarms of wild dogs, duplicate-creating fast psi-dogs, invisible and fast swarms of bloodsuckers, tank-like pseudogiants, aim and movement-hampering controllers, invisible telekinetic poltergeists, and fast boars. Almost all of these can take a ton of damage before dying.