How to take on a Rex 101:

You need:

A Pike

A Spyglass.

Some armor to keep the other predators from killing you first.

How to kill one:

Step one: Look for a Rex

Step two: Run from the Rex. If it doesn't chase after you go to step three,If it does go to step four.

Step three: Wait for it to attack random creatures and follow at a respectable distance. If you come too close to to step four.

Step four: Run from the Rex. Go to step five If you survive.

Step five: check with spyglass to see Rex health.

Repeat process until Rex has less than 100 health left. Go to step six.

Step six: Stroll in toward the Rex and casually stab it twice with a pike.

You've killed a Rex. Hmm? What? You want to tame one? But why??? :)