The rock scaling allowed after the TLC phase 1 makes using rocks and even large cliffs illogical, if the cliff has even one section that’s less than a 65 degree angle, that Rex WILL climb it. Even after they nerfed it a little, it can still walk over most anything. I would suggest stone structure pens with a ramp to let it in, it will also derp through even small gaps between the 3 Dino gate strategy, the moment you realize that, it will have escaped and be coming at you.
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.