What you want to do is use a cross now or a normal bow it works just bring a ton of narco arrows and some normal narcos to keep it knocked out make sure you have some other ok battle Dino with you so you can kill what ever is there a use that to tame it
Before you go hunting for meat MAKE SURE YOU PUT UP A SPIKE WALL TO KEEP DINOS YOU DONT WANT OUT
That is very important bye
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.