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.)
The best way for me is to get dino gate ways and place them in a circle but leave one hole open to lead the rex in it so get a tribe mate and send him in with the rex trying to eat him and once it's in place the last one behind it
Be a safe distance from the Rex then strike! When you knock it out make sure you have enough narcotics (if not make sure you have a raptor to get home quickly!). It’s easiest to hit it with a long neck rifle in my opinion
Create a 3x3 mobile taming pen 3 walls high on a raft with a Dino gateway on one side and ramps on the other. Use a flyer to lead it up the ramp into the pen then move it to a safe area to tame. I got 2 150s without a single issue.
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.
In the dododex it says,you will need 700 narcoberrrys to tame I with raw meat,but I tamed a lvl.36 Rex with only 300 cauze I dont really have much narcotics(I'm lvl.39).I play on single player and now I dont know if the dododex is extremely wrong or my game has something(I didn't change anything in the settings to make it easier)