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.
as u can see darts are the most effective. load up, have a friend pick u up in a bird, and fire away. i suggest the spike wall tactic just to be safe if you want a perfect tame! we got ourselves a 150 female earlier today and woke up at a lv 220!!
If you fly to where the beach touches the snow biome on the west coast of the island there's a huge rock you can tame them easily from. Nothing naturally attacks them when they're KO'd so you can just leave it there
I tried the triangle formation of metal dino gates as per one of the top tips but that didn't work. What did work for me was a four wall box of stone behemoth gates and then kiting the rex into there with a flyer and then closing the rex inside. Make sure you placed the gate box near a high boulder so that you can land there and tranq the rex from above.
Once knocked out, you can honestly get away without taming spikes if it's just a good Rex. Nothing naturally attacks them in the wild if it's not fully tamed. You don't need to waste material and sit next to the rex if you're kibble taming it for 3 hours. Just knock it, fly back 10 mins before torpor time and make yourself productive. Unless you're on a busy server with a lot of traffic