a. Get 3 stone dino gateways (not behemoth)
b. Place them in triangle shape but leave one side for the rex and put a large bear trap in the center.
c. Once the rex steps on the bear trap, go around and place the 3rd gateway
a. Place 3(minimum) gateways(small) like how you would in domino, with a small gap just enough for you to squeeze through in-between gateways.
b: Place dino gates in the first and last gateway, leave either one open.
c: Let the rex chase you to the trap, once it's inside and chomped you once, run to the door where it came from and close it, then exit through the gap.
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.)