Actually, the skin of a T-rex is so tough that you can't shoot them. You have to trap them!
This is the deal!
You dig a large hole with vertical walls, and you then build a HUGE fire in it. While you wait for the fire to burn down, you find some peas. (The T-rex loves peas.) When the fire is burnt down completely, and nothing but ashes is left, you carefully place the peas around the edge of the hole. Then you hide somewhere in the surrounding terrain.
When the T-rex comes up to the trap and bends down to eat a pea, you just sneak up behind him and "kick him in the ash hole"!