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.)
Use a tame that can take a beating, like a thyla, preferably an imprinted one. Then use a really good crossbow, they reload quicker so as your tame takes the beating you don’t have to worry about it being killed as you are pelting the rexy with arrows. Then once it starts running you can take your time pelting it with arrows so you don’t kill the Rex you are taking.