Step 1: go to the server settings before loading the game, and set Dino Damage to zero

Step 2: start the game

Step 3: find the Rex

Step 4: punch it so many times until he passes out... he may run away so get him stuck in a wall or something

Quick Notice step 4: it may die if you punch it to many times

Step 5: tame it

Step 6: enjoy having your OP Rex or not

This is how i got a Rex :) :) :)