After Miranda meets with her contact on Illium, She takes a car with Commander Shepard down into the cargo areas. At the same time, the A-61 Mantis Gunships led by the merc leader are dropping troops into the cargo areas. When the car is in view, the Eclipse mercs open fire on the crew. Once the Merc Leader confirms that Miranda is in the car, he orders his men to hold fire. Upon confronting Miranda, he explains that he was ordered to give Miranda one chance to walk away before things get ugly. Shepard can snap the Merc Leader's neck while Miranda shoots an overhead fuel container taking out his mercs, or allow him to continue talking, which results in him being killed by Miranda and a firefight with his troops.
The Merc Leader is essentially an Eclipse Vanguard. He has barriers and health and is equipped with an assault rifle. The Merc Leader also uses his biotics to deal damage. It is recommended that you deal with the Merc Leader first before you take out the other Eclipse mercs.