As a Ghostbuster, you will be pitted against an assortment of supernatural threats to your well-being. The basic system for combating ghosts and the like is described in a three-step process: "Sap 'em, Cap 'em, and Trap 'em".
The first step once a hostile entity is encountered is to sap its psychokinetic energy (PKE). When you aim at a target, a gauge appears describing its overall health. Use the tools available to you (such as the Blast Stream) to attack it and drain away its energy. If the target is corporeal, this is the only thing you need to do, as it will fall apart once all its energy is sapped away.
For ghosts and unstable entities, the PKE gauge will eventually turn red. When the gauge turns red, it's ready to be captured. Switch on the Capture Stream (when targeting a weakened ghost, using the Blast Stream automatically engages the Capture Stream). Unstable entities can be destroyed by grabbing onto its key element and yanking it away. Ghosts need to be wrangled so as to stun them long enough to trap. For this purpose, a Slam meter appears when a ghost is in the Capture Stream. It slowly builds strength, and you can accelerate this by fighting the ghost; as it tries to flee in one direction, yank it back to boost the meter. When the meter is charged enough, you can then Slam the ghost against the ground, walls, or anything handy to stun it.
Once a ghost has been stunned, it's ready to be trapped. While keeping the ghost in your Capture Stream, throw a Ghost Trap at its feet, then carefully guide the ghost over the aiming beam the trap provides. Position the ghost correctly and the trap opens and begins to draw in the ghost. The ghost will attempt to escape, so continue to wrangle the ghost into the center of the cone of light until the trap closes on it. If the ghost escapes, simply recapture it and try again. Ghost traps will reset themselves after a few seconds to prepare for the next ghost, but they still need to be retrieved once you're done.
Health, Heat, and Teammates
The right side of the Proton Pack measures your overall health by displaying horizontal green bars. As you are attacked, you lose health and your vision will begin to blur; this health can slowly regenerate if you find refuge or clear the immediate danger. If the health bar goes completely out, you'll be incapacitated; if a teammate is available, they'll try to help you back up. Your display will also indicate to you if a fellow Ghostbuster has gone down (red circle, eyes closed) or has become possessed by a ghost (white circle, eyes glowing). It's in your best interest to help incapacitated Ghostbusters back to their feet and to slime possessed teammates to get any possessor ghosts out of them. If all available Ghostbusters (including you) go down or are possessed, the mission is failed.
As you attack with your weapons, your Proton Pack will build up heat; this heat level is indicated by vertical red bars on the pack. Some weapons produce more heat than others, but the Capture Stream does not build heat so can be used as long as necessary. If the pack gets too hot, you will hear a warning beep. Continue to attack and the pack will automatically shut down for a few seconds to vent and cool off. If weapons are not used, this heat will slowly dissipate, but you can also manually vent the Proton Pack to quickly cool it off.