first or second t-rex? For the first t-rex there is a big rock with small t-rexs spawnig behind it. I stayed at the seam of this opening where the small t-rexes didn't attack you and the t-rex didn't come in. Shoot him with pulse rifle or whatever till he goes down then knife him. For the second t-rex i ran to the top left which has a sticky then to a crate to the top right which has a rocket luancher. Run around the crate shooting him in the butt till he goes down then knife him.