anyways: run down the core (giving yourself covering fire as you run), all the way to the bottom, now, if you're lucky, only a couple of units will follow, maybe a visceroid and one of the stealth guys, and petrova. go right into the busted vent that you came out of, and then reload, then step back out into the open, and kill the units other than petrova, just ignore her untill you've killed everyone else (don't ignore her when she shoots tho) once everyone else is dead, you can focus on pertrova, and she does this psycho jumping-across-the-room thing, that can get really annoying, but just keep pumping rounds into her while dodging her attacks, she spends a couple of seconds recovering from her jump when she does, thats when you can hit her the hardest then, once she's dead, just go back up to the top and kill the remaining two or three guys and rescue sydney then there's some more stuff you ahve to do, but i won't spoil it for you