There are numerous special enemies that can be quite a pain. You can make it easier to see them and take them out by using the shout-out command while aiming at them. Here are some of the enemies and what they do.

Taser: Police who wear 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 to aim 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 Very deadly 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 immediately.
Bulldozer : Big bulky 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 take them out.