Here is how I did my one.

1. Go to Carno island.

2. Locate your Rex.

3. Use a raptor to make it Afro on you and run to the small cliff on Carno island.

4. Make sure you are at the top of the cliff and your Rex is at the bottom and shoot at the Rex

5. If the Rex somehow manages to get up the cliff. Just get on a raptor and run of the cliff(don’t worry, it’s not high enough to take too much damage). Run around the cliff fast enough so that when you get to the top the Rex is still at the bottom.

6. Repeat until knocked out.

P.S. there are better ways to do this like using a trap but this is how I did mine so this is kind of a cheap way of doing it.

Anyway happy taming!