Close Quarters Specialists are the Seventh Wave's main heavy soldiers. They are a forced to be taken with extreme prejudice, with heavy armour and masks and a SPAS 12 or Remington 870 shotgun. Strangely enough, the ones encountered earlier seem to be more strong than those encountered later. For instance, three headshots are required to kill one in Treneska Border Crossing (Your first encounter with them) (Two to knock of their mask, and a final one to kill them), where as the ones encountered in Graznei Bridge only require one.
If encountering a group of enemies, with a Close Quarters Specialist amongst them, they should take priority. DO NOT engage in close quarters unless, A) You have a shotgun yourself or, B) If they have been knocked down. Try and keep them away from you, running backwards while firing at their heads is often a good tactic to kill them. If encountered at long range, they will just stand their pathetically, trying to take shots at you, or will try and find a way to your location, which takes sometime because they are quite slow. They have heavy armour and reinforced facemasks, making them quite relisilent to bullets. Concentrated fire on their torso, head or a close range blast with a shotgun will knock them onto the ground, leaving them vunerable to another blast or a melee attack. This also indicates they are at half their usual health. If so, either continue to shoot them, or, if you're feeling cocky, run up and bash their faces in with a melee attack. Grenades also kill them in one blast.
Sometimes a Close Quarters Specialist will fire at you with their shotgun very rapidly, causing you to die very quickly, this glitch can sometimes be extremely rare, but it is known to happen near the end of Vratska Dockyard.