T-rex

I think there are a total three T-Rex encounters.
The second one all you have to do is this.
Keep Shooting at it.
Plain and simple but you have to watch out because for some reason they like human meat!
There is a big tree in the middle with a little area underneath. I found it best to just sit under the tree firing as much as I could. Also try throwing a couple grenades at it when it gets close. (Beware it will sometimes shove it's head inside the bottom to the tree so try staying on the opposite side the T-Rex is on.