If you’re in the snow biome, have a decent and fast argy, and a safe place to k.o., this is a pretty simple tame. Get on a ledge, knock it out, and then just kill anything that gives prime. Argies, purlovias and mammoths are just a few of the animals that drop prime. Keep loading it with prime and eventually you’ll tame the dinosaur.
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.)
For me, I just stood on a higher land than Rex, but not like the stupid people and just a high cliff. I stood atop my battlements( for those who don’t know, are massive walls higher than any creature and have an extremely large amount of resistance.) and shot at it a few times and had it done. It could easily be a high structure instead, but a strong and less necessary one, but none of this cliff stuff or high rocks. You do that, might as well go at it and try punching it to death.
Mine was really easy to tame, I just used a pterosaur to get on a mountain and used 10 tranq arrows and she was out. I only needed 28 raw meat and 1 biotoxin before she was tame. Keep in mind however that I am on mobile so it’s most likely different if you’re on something else.
One thing you can do is get a flyer I recommend a argue or pteranadon and get a truckload of tranq darts and meat it worked for me plus I wanted to get revenge for the time it killed me my tamed and base