Make your way up to the rooftop of Duga Substation and youll spot three glowing psi-emitters arranged in a triangle pattern. Grab a shotgun first, since its the most effective against these devices. When the fight starts, take cover behind concrete barriers and focus on destroying the emitters one at a time, while watching for Fausts grab attacks. Once you've cleared all three emitters on the roof, look for the red module on Fausts hip, thats his weak point now. Stay mobile and keep your distance, as his grab attacks have a slight delay that makes them easier to dodge. Tip: destroying the emitters quickly is crucial, since they power his clone abilities making the fight much harder the longer they stay active.