Assassin

Assassins are individuals who have been hired by others to kill a specific target. These people are usually equipped with pistols in order to keep a low profile.

If Shepard decides to go looking for Harkin first, rather than going to see Barla Von, when Shepard tries to enter Chora's Den the first time, two Assassins will be waiting in ambush.

Saren and Fist hired some Assassins to take out Tali'Zorah nar Rayya to prevent her from giving her acquired data that will expose Saren as a traitor. They set up a situation where Fist said that Tali would meet the Shadow Broker but it was a trap. Shepard and the squad arrive just in time to save her.

Tactics
Because of the area that these enemies are faced, they are in a confined space that offers little cover. However, these enemies don't have much in the way of shields or health, so they shouldn't put up too much of a fight. They are only armed with pistols so you don't need to worry about then ripping through your shields too quickly. Keep in what cover you have and you should have no problem taking them out.