If you’re like me and don’t have anky’s or dodek’s to grind for materials, just get your movement speed up to 145%. This allows you to outrun them easily. Also, I play on 0.5 difficulty, so the max level of creatures that spawn is 60. I also have taming rates doubled. Just run away from the rex and periodically turn around and shoot it. I like to do this on the east side of the island by the big mountain. That way the beach is fairly safe and there is lots of room for the rex to start torpor running. The water usually isn’t a big problem. I just tamed two rexes this way!