When taking it, make sure nothing can get to it. Put spikes or gates up around it to make sure it doesn’t get killed, it doesn’t help to get the Rez killed. As for knocking out the Rex, the simple and safe strategy might be to use a flier and fly above the Rex and have your buddy Tranq it, this way you won’t get hurt. Just make sure you don’t don’t run out of stamina.
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.)
You can use the normal darts or arrows. But the fastest way to tame a Rex .
Get under its belly as fast as you can and rub its belly, instant tame :) all you new player try it out and you'll be riding like a king like us vets !!
Create a 3x3 mobile taming pen 3 walls high on a raft with a Dino gateway on one side and ramps on the other. Use a flyer to lead it up the ramp into the pen then move it to a safe area to tame. I got 2 150s without a single issue.