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.)
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)
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
If you want to tame one of these pretty early, its actually quite easy that you can even do it on foot. Just a crossbow and atleast a stack of tranq arrows is enough (I recommend 2 stacks just in case). Just make sure to lure it into a nearby herbivore and it'll attack it instead. During that time just shoot it until it pass out and there you go, all you have to do is to feed it and wait.