Dunno if you've managed to do it yet but I had the same problem. When I've tried to do it when Lara is stationary or just running forward/backwards, although I've pressed the correct buttons the dodge did not happen and the sight was still pointing at the centaurs belly.
What you need to do is aim (R1 on PS2) at the centaur and move Lara left/right. (Her arms/guns and face will be facing the Centaur but her body will be going left/right) When the Centaur charges keep pressing the left/right button along with the aim button and at the last minute press the O button (PS2). That way the dodge works properly and the markers will join on its head so you can get a head shot.
Hope that helps 'cause I was stuck on that a long time too and know how irritating it can be!!
