To beat the T-Rex without dying (or even losing any health if done correctly), you need to stand in front of one of the spike traps. Then when the T-Rex charges at you auto-aim and keep firing at him, then dodge just before he hits you (in the same way as how you can dodge any other creature that charges). This will give you a chance to shoot him in the eye when time slows down, do so will make him run strieght into the spike trap. You need to do this 3 or 4 times to kill it. You can also do this near a wall, however it will do less damage than one of the spike traps. This will also net you an achievemens as long as you do not die against the t-rex at all.