I found this difficult, too, but you need to shoot the ‘pods’ at each side of the creature (the ones that spew the acid). Once you have shot them all off, the creature will drop.
Lara seems to keep aiming at the same one all the time, so I did what one person suggested and pressed the square button on the controller to change the target (on PS2…I don’t know what the button would be on the PC keyboard).
Once the creature drops, it will change into a smaller version that is much easier to dispense. Once this creature drops, walk towards it and you will be rewarded with a cut scene. You may have to stand over it for a few seconds for it to begin, so be patient. 🙂
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