In the first area, simply continue shooting at him (preferably with the revolver) and toss grenades to force him out of hiding. After losing some health, he will toss a smoke grenade and run away. Grab any supplies and take the M82 or keep the revolver. The second area is slightly wider and Metrac will run around on a catwalk to the right and top. Shoot the valves on the right side when he is over there so fire shoot out of the valve, blocking off that area from Metrac. The best way is to trap him on the right or in between the two fire jets (use the object lock). Metrac will retreat again after taking enough shots and go into a third area. Resupply, then follow him. In this area, it is best to use the M82 just because it fires more shots, which is good since you don't have much time to be precise. Metrac will be in a mounted machine gun turret, which is heavily armored. If ou sit there and shoot at the turret, you would probably run out of ammunition (although you can destroy the turret this way). Instead, shoot the generators on either side by throwing grenades at them first, then finishing them off with the M82. The gun will power down and Metrac will get out to do the same little dance he did in the other two areas: fire some shots, then launch grenades, reload, and repeat. Shoot him the same way you did before, but be careful. All the boxes except the ammo boxes and the two crates that are front of you can be destroyed. When he is launching grenades, wait until the grenade is in the air, then move so the grenade lands in your previous location and doesn't hurt you. Repeat this strategy to eventually win.