if u havn't figured it out wat you do is; run all the way across the bridge to the door from where you entered and leave the area, and turn around and re-enter the area, he'll be across the bridge and run at you. What you do is bring out your sniper and snip his face, then turn around and go through the door again and re-re-enter. Repeat this process untill he drops the plagma sample.

Semi-Auto Rifle, TMP and grenades all to the face make easy work of him.

Alright, run to the end, stop just before the last line on the ground. handgun the
barrels. Go towards them.

[cutscene]

Grab the stuff, and turn. RLH to the other end, then get your rifle. Aim at his feet, and
keep shooting. Throw a grenade at him if you get far enough away.