Battle! Paine vs Sam Fisher

Well, the Barrier doesn't negate all physical attacks, it just significantly diminishes their effectiveness. Again: Shock, Snipe, Shock, change locations. Rinse and repeat to absolute death.

Well, yes, that was my point, that the sniper rifle would not deal terminal damage to Paine in one shot.
While Sam is changing locations, Paine can be healing, or restoring by the pre-cast regen spell.

So, let's see. Sam sets up in a dark corner, and shortly notices Paine coming around the corner. He either distracts her or smoke screens her in, which leads to a Stuning/Shocking device. He then Snipes her head for some decent damage, then Air Foils her to a dizzying state of incoherence, which gives him more than enough time to use his Assault Rifle for a good 15 rounds (Which will take less than 1 second). He then procedes to Stun/Shock her again, and after doing so, he throws 2 grenades, and changes locations, but not very far away.

Sure, if the head shot Sniper, 15 Assaults, and 2 Grenades don't kill her, they will at least damage her pretty well. Now, she has Regen on, which as we all know is a decent spell, but at times, definitely doesn't recover fast enough. I'd say that Sam is in such a location that Paine gets half, at most, of her life back, which is when his viscious Stunning and tactical arsenal is unleashed again, and again, and again, until Regen has nothing to but death to unsuccessfully Regen.