Capture a T-Rex (Easy Way)
To capture a T-Rex the easy way you have to use the call box and the 9mm Rail Gun. Set the call box in a big open space and wait. If it works a whole bunch of T-Rex will come.
If you use the 9mm Rail Gun one shot will kill them.
Catch a Styracosaurus (Easy Way)
To catch a Styracosaurus the easy way you have to go into the woods, to the part where there are swamp plants. This is normally where they have their nests. When you find one shoot it once.
It will take a few hits to kill with a Tranqulizer. Now get it to charge you and then run away from it. If you face it, it will grant you a straight shot at it.