secret? *there's a secret to this? *oh man, i have to know this. *but while i'm waiting for someone to answer, i'll tell you how i manage to beat her.
stand with your back against one of the walls on the side of the long hallway so you can see Sniper Wolf's laser sight waving around looking for you. *when the beam disappears, quickly face down the hall and equip the PSG-1 and pop a Diazepam (i mean make Snake take a Diazepam) :)
if you don't see her within about five seconds, un-equip the PSG-1 and hide around the corner again with your back to the wall so you can watch for her. * sometimes you can see her running along the walkway at the end of the hall. *keep trying until you perforate her forehead 5 or 6 times.