Ok,
One, DON'T sack Lara.
Two, I had a hellova time tryin' to beat those evil horses, too. I'm usin' PS2 so I don't know how the X-Box controls work but I'm thinkin' that performing the tricks is simular to how the Playstation works. Anyhow...
Keep shooting one of the centaurs to get him enraged, just like the T-Rex.
If possible DON'T put the guns away as it'll take you longer to enable the Adrenalin Dodge [AD].
When he's enraged he should stop and stand up on his hind legs.
Enter; the slow motion scene, this is the AD starting up.
Now this is the important part, STAY CALM and DON'T SHOOT him, but keep your guns out ready.
As he charges towards you, you'll notice that he lowers his shield so that he can ram you down.
As you see his shield lowering, brace yourself for sliding off to the left or right.
Before his shield is fully lowered, move FAST and slide to the left or right.
Get ready with guns still at the ready. You MUST never put the guns away once the AD has started as this will break the AD cycle and you'll have to start enraging the centaur all over again [it took me a while to figure that out

].
Still holding the guns at the ready, wait for the two grey target points to link into one. It'll turn red, flash and you should hear a couple of beeps alerting you to shoot one round.
Now the centaur is stunned for a few seconds, move FAST and grapple his shield and pull it away from him. You'll have to be super fast to pull it off.
Now repeat the same for the other centaur. Also, if the remaining centaur locks onto you with the green beam, pick up the shield to protect yourself and reflect the beam back at him. He'll turn to stone so move FAST and shoot his ass. I found that running around him whilst shooting non-stop made him shatter into driveway gravel more quickly

Another trick is; if you find yourself unable to shake free of the green beam, do a non-stop backflip away from the centaurs as fast as you can. On the Playstation 2, this gives you time to 'shake free' from being turned into a statue. The trick is to move the left analogue stick from left to right at lightning speed. By doing this you'll shake off that clay exterior.
Hope all that helps.
Hooked on Tomb Raider in spite of not being able to figure some things out. Hense the username.