1. Build a pitfall trap (2*2 or 2*3) build up each side at least two walls tall out of door frames (wood works, but for anything over lvl 50 i recommend using stone). Add a ramp going up one side into the trap

2. Load up on tranq ammo i recommend being over prepared in example if your using a crosbow 100 rounds outta to the trick but for a bow bring 150

3. Set a bed outside of the trap with a storage box to put your taming gear (weapon, ammo, sleeping agent(i.e narcotics), and taming food)

4.lure to trap, if your a low level i recommend riding a dino into the trap hopping off and running. If you have tames use a flyer to fly over the trap, or i prefer using an iguanadon to jump over the pit trap from the top of the ramp(it dose clear a three foundation gap at 108% move speed).

5. If you died spawn on the bed Asap, otherwise get into your taming gear and begin shooting the therizinosaurs with your tranq ammo being weary to not over shoot an loose tameing effectiveness.

6.maintain its torpor by feeding it the prepared or gathered sleeping agent. On standard single player settings it may talk a very very long time to tame if you use berries like me so keep a watchful eye

AFTER: when you have the therizinosaurs, you have the vegitarian rex. It is a berry eater so yo wont get any fast health by eating. But from my experience anything lower in level than it on the island is pretty much screwed until its dealt with. Along with its decent combat its a fantastic gather of many things. Hope this helps