T-rex are not good that they can spot you easily. Weak spot for them is around their eye.

Here are the tips for hunting them:

T-rex sometimes stands up high looking for you, it roars like the sound of a Ceratosaurus if it spots you.
When it is far, use a sniper, but when it is running towards you, use a shotgun (I recommend a double barrel shotgun)
Look at your radar, relocate to the sea or lake where the T-rex is on shore and call it, then it will swim to you, then shotgun of any kind will work.(Doing this on RAVAREN'S BRIDGE is much easier.)