Have a tribe member hold you while on a Flying mount (Pteranodon,Agrentavis) and hover above the Rex and shoot it with tran'qs. When it's down, get spikes and surround it with them. Make sure you have a way to get in and out for any reason.
If you use the bear trap strategy which is if you lead it into a Dino gateway that is shaped in a triangle and with a bear trap in the middle (Large Beartrap) and when it is stuck you must put a dinogate behind it to trap it and then knock out. (hope this helped and "If" I win could it be an Xbox voucher, thanks.)