First go on a cliff and since you would be on the top of the cliff you can shoot it without getting hurt. Then once it is knocked out get some pulmonoscorpius kibble and raw mutton you will need at least 20 bio toxin and then just wait for it to tame. ?
My brother has tamed a lot of these beasts, he only keeps running around some rocks while shooting him tranq arrows and a few moments later he has a brand new tamed Rex;) if you want to tame this you don’t really need to mount a fast Dino, you just have to make sure that the rest of the place is secure
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)
Everyone says: do IT in a mountain. But you have to look out tgat the rex cant climb up. They are good climbers and have a large hitbox. I've lost my saber who couldnt climbe because het was too small and died to the rex, (i followed him a couple seconds later)