It’s really not that hard a tame. Trap it with stone gateways and a bear trap. Knock it out with maybe over 100 tranq arrows. (Which aren’t as hard to get as you may think). And BOOM! You have your self a rex. Oh yeah and feed it some kibble, for best taming better stats AND less time for taming. I would say it’s a medium tame.
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)