There are several special enemies that can prove quite tough. You can make it easier to see them and take them out by using the shout-out command while aiming at them. The following are a few enemies and their behaviors:
Taser Police wearing yellow stripes. You will hear him charge the tazer before firing at you. If shot, you will be unable to do anything but look around while your character randomly fires at them. Try aiming at them in this state. If tazed for too long, you will go down.
Shield Black officers who have riot shields in front of them and try to move up. They can be flanked easily and taken out from the side and back. Bullets will not hurt them in the front. They are incredibly tough when in a hallway. Spec-Ops Agents wearing green night-vision goggles. They will handcuff you to the floor if they get too close. It is recommended that you take them out as soon as possible. Bulldozer Big buff enemies covered in thick armor. They do massive damage if they get too close and require a lot of bullets to be brought down. All fire should be focused on them the moment they are identified. Headshot will help.
Sniper Although not a "Special Unit", they are deadly. Two shots will drain your health. Either avoid them or eliminate them immediately.