The easiest way to deal with Mama Rex is to pick up the key, roll, and jump out of the nest. Then run forward along the right cave wall as she comes around the corner from the left.
Run along the shallow edge of the pool. Then head for the deeper water in the far right corner, between the little island and the gated alcove with the second switch.
If you stand there in the waist-deep water, you can shoot the T. rex with pistols, and she won't be able to reach Lara. It takes several minutes of sustained pistol fire to bring her down, but there's no danger involved. If the dinosaur wanders out of range, just step out of the pool to get her attention. Then wade back in and keep shooting.
When the T. rex falls, you still need to open the gates in order to get out.
Run across the open area with the raptor carcass and into the alcove with the first switch. When you took the key, the gate opened and the nearby lit, showing you where to go.
Pull the switch to open the gate on the other side of the cave. (The gate next to the switch remains closed for now.)
Run back across the cave toward the pool.
The torch near the gate is now lit, making the gate easier to spot. Enter the alcove and pull the second switch.
This opens the third gate, back at the first switch, letting in another raptor from the upper level.
Again, you can wade into the pool and shoot at the dinosaur from there. If it loses interest, step out of the water briefly to get its attention. Then move back into the pool and keep shooting.
NOTE: If you slid down the embankment before killing the 2 raptors in the upper area, they will also come down when you pull the second switch.
When the raptor is dead, return to the alcove with the first switch.
Pick up the flares and small medi pack on the stairs if you didn't get them earlier.
Then climb the stairs to the upper level and leave this area through the tunnel in the corner, where you came in.
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