Load into a single player game. Upon loading press the menu button to pause the game. In the console line at the top of the screen type in "summon gigant_character_bp_c" without the quotes and press admin command. Quickly unpause and then pause again the game so that you can see the name of the trex with it's level and then in the box type "forcetame" and press the admin command button. This will tame the dino and you will have your achievement.
You must have your cursor facing the dino in order for the tame to work. This is why you must unpause and then pause again.
If you are having problems with this you may want to type "god" without quotes into the box and press command to start god mode so you don't get eaten alive.