The key to defeating the Amanda creature is avoiding direct hits by staying on the move and using the large stone temple fragments located around the perimeter as cover. Ignore the pillars, they are too skinny.
Be aware of the creature's location at all times and hide on either side of a fragment as necessary. If you are properly positioned you will take minor or no damage from the creature and yet be able to use Excalibur to attack, and you should be firing almost constantly whenever the creature is anywhere in front of you.
In fact, if the creature is lined up just so, you can take one baby step back from a fragment, and fire Excalibur THROUGH and ABOVE the fragment to hit the creature -- and still be invulnerable!
Just be careful not to venture too close to the edge or you'll fall off and die. Also, don't stray too far from the creature when its health is low, otherwise you may not be close enough to stab it with the action key before it recovers.
Can't find the Grapple? Where Zip is, on your right there is a laptop on the floor go near it and press action. A safe will open and there you have it! The guns are in Lara's bedroom, there's a wall with two swords that you pull down and the circle thing will open up. The light is in Alester's study and the binoculars I think are somewhere but I have found them...once.