You'll want to be extra careful around these zealots because they're different from your typical enemies. These soldiers are controlled by psychic emitters, making them fight way past what normal humans would endure (keep your distance since they'll push through injuries that would stop others). When you're near the Nuclear Power Plant area, they become particularly aggressive and dangerous, so always have an escape route planned. Something interesting - they actually ignore anomalies due to their conditioning which you can use to your advantage by leading them into hazardous areas. The closer you get to their territory, the more you'll notice their unnatural behavior and relentless attacks.

Watch out for their recruitment tactics too, since they use those psychic devices to forcefully convert stalkers (maybe pack some anti-psi equipment if you have it). You might run into some Noontide members who used to be Monolith, and while they've broken free of the mind control, most stalkers don't trust them much. When fighting these guys remember they don't respond to normal tactics like suppressing fire or showing force, since the brainwashing makes them pretty much fearless. A good tip is to always engage from covered positions, because they'll keep pushing forward no matter what you throw at them.