If you already have a rex with a saddle and high health (mine has 20k) then you can simply walk up on your rex, get off so that you're still on it's back and then shoot the wild rex while your tame tanks the damage. I have tamed a few 150 Rex's this way. You can also use this method while kiting the wild rex, shooting it while it charges you and then move again.
I recommend using a scorpion to tame it, pump all levels into health on the scorpion. Then just attack the Rex but with 10 to 20 seconds in between hits, DO NOT pump anything into dm. Also this is what I use to tame literally everything, and it doesn’t even cost anything. Pls put this up in chat for everyone to go “oh that’s not a bad idea” 👌😄👍
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.