The most important thing to remeber is to constantly shoot the T-Rex and when it leaves focus on taking down those pescky pteranadons, but when you hear the T-Rex coming Stop attacking them they will leave and quickly shoot the T-Rex before he gets close to you. Make sure to once again take any pteranadons down before they kill you.
Once you beat the one T-Rex there will be another. Note that the second T-Rex likes to smack you with its tail. So for this one it is extremely important kill it when its charging or at least shoot it while its charging, to avoid a tail smack