Trapper offers this quest as soon as the player arrives in Yanov for the first time. He needs a group of Bloodsuckers eliminated from the swamp east of the Smoothie (Fen) anomaly, near the cement factory. The threat they pose to stalkers in Yanov is becoming a source of concern.
It's safer to approach the site from the east: the Bloodsuckers can be lured one-by-one into the relatively large open area just east of the anomaly yet far away enough from the target to avoid dragging more Bloodsuckers into the battle. There are usually four Bloodsuckers to kill regardless of difficulty setting.
The mission objective on the minimap locks on to one Bloodsucker until it dies, and then it tracks the next. This is great for tracking down far away Bloodsuckers, who tends to lose interest in the player's antics and wander around. It's also good for tracking down cloaked attacking Bloodsuckers, however the minimap updates with the location of the creature once per second - much too slow to be of use in combat. It's better to track movement by looking for the ripples in the water as the mutants try to invisibly flank. The second factor in the fight is terrain. A provoked Bloodsucker will follow the player surprisingly far. Being backed up into the northern boundary of Yanov is disadvantageous, and accidentally backing up into the anomaly to the west of the mission area is quite the blunder. The fight isn't so difficult that the average player would need the assistance of other stalkers, but the cement factory usually has a squad or two of either Freedom or Loners, but sometimes Zombified Stalkers.
Despite shotguns being for obvious reasons "mutant weapons", the best weapon to deal with these bloodsuckers is any weapon with good scope, at least with 4x magnification (sniper rifles or upgraded IL 86/GP 37/FT-200m) and plenty of ammo (30 rounds should be enough). Just go on top of Cement Factory and snipe all mutants from there. If you are good shooter, you shouldn't have problems with dealing with bloodsuckers.
You can also combine assault rifle with shotgun and kill at least one bloodsucker from distance and try to kill rest with shotgun to avoid being easily overrun. Even that bloodsuckers doesn't have much health, they can still do heavy damage to you, especially when they are in pack of three.
However the job is done, once all the Bloodsuckers are dead, speaking to Trapper will give the mission complete. Trapper will now offer the player the next mission, Hunting: Unidentified Mutants.
- The swamps are particularly graphics-intensive. Between the Bloodsuckers, muzzle flashes, anomaly, and the water effects, swamp combat can strain a computer's resources. If the fight is proving especially laggy, lowering graphics settings may help.