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.)
One of the best and most efficient ways to tame one is use a large wood signpost or a metal signpost literally one of the best traps I've used get it to by a decent tame that has good health and try to place a bunch of these in front and around it to trap it and back away and it will just sit there and bam rex tamed
Dont be scared of them, get a flyer and make sure to have a high health and stamina, fly at him (make sure its only one or your dunso) and let him take a bite of your flyer once in a while then lead him into a square wall of dino gates with a trap in it. Once he is stuck quickly close up the open gate, laugh at him and shoot him (alot)
Is easy just keep it for water and if you have a high advantage you are sure to tame it I got my first rex by doing this and I got it when I was level 25! Now I am level 30 and my rex is level 20 and make sure you have some Prime meat if have Tranquilizer arrows you are set!