Once you have the Rex knocked out you are better to just start the timer and leave it because nothing aggros to a Rex so nothing will attack it other than other players. I usually knock it out and go back to base make the kibble etc or do whatever you need to do for two hours then go back narcotic it up and repeat until the starve timer is ready and it is ready to feed (: same goes for a giga! Enjoy!
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.)