Is easy just keep it for water and if you have a high advantage you are sure to tame it I got my first rex by doing this and I got it when I was level 25! Now I am level 30 and my rex is level 20 and make sure you have some Prime meat if have Tranquilizer arrows you are set!
To tame one make a trap out of wood gates, stone if a higher level, that looks like this (from above) /_\ afterwards, put a large bear trap in the middle. Then, using one last gate, fill in the gap on the top. (Obviously you can rotate the trap making the gap on the bottom or top.) Afterwards just knock it out! Good luck taming! 🦖
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.