they are not as strong as you think you just need tribe mate whit armor (recomendet is metal but chitin is ok)and shield and while he distracts the rex you can knock him out whith crosbow but it only works on lover lvl rexes i think the highest you can use this method on is lvl 50 but if you have more teamates you can do it on higher lvls
its good for geting rexes early whitout wasting lot of resources ( also bear traps help if your friend needs to change armor or shield)
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.