It may be difficult to get some time when the Rexes are not attacking you. However, the very first time you fight the Rexes, there is a ledge which you can climb onto so they cannot attack. However, once you are up there, you will be attacked by the flying dinosaurs. They are easy to defeat. Once you are up there, go into fury mode by repeatedly tapping Y until he smacks the ground. Once in fury mode, attack the Rexes one at a time. After you have hit one of them three times with A, they will be temporarily knocked out. You can then perform a finishing move by repeatedly tapping B or A. If done correctly, Kong will snap the Rex's jaw in two or throw it.