The boxing method mentioned by others is very useful! I trapped a level 92 Rex in the box and it took under 20 tranq darts with the basic longneck rifle to knock it out (most were body shots before I started aiming for the head, I’m not sure if headshots increase torpor quicker). I killed some baby dinos beforehand and had my hyeanadon hold the prime meat in the craftable meat punch saddle. The tame bar goes up by over 20 percent for prime meat each time it eats, and 9 percent if it eats raw meat. The guide says the Rex’s TORPOR reduces quickly, but it DOES NOT. I did not need to put any narcotics into the Rex throughout the entire taming process. Have fun, you got this!