I’m taming a lvl 5 right now and it took 22 tranq arrows (crossbow) to knock out and I’m using a mixture of raw fish, raw meat and raw prime to tame! I choose this way because I’m level 39 and not very good! (Please tell me if I’m doing good with the whole lvl 39 and a lvl 5 Rex)
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.)
If you bring your high health Spino, hit the Rex once and it will just get distracted attacking your Spino instead of you. Just dismount and tranq it then when its running away just chase it as you tranq it then it shall get unconscious. My plan became successful. The rex i tamed was a.lvl 10.