How to defeat T-Rex:
When you start the level, you will meet T-Rex in an intermission sequence. There's a quite easy way to defeat this monster! Press A and Y and keep hammering it. Press the X button if T-Rex tries to attack you. Thus you can make him fall on the ground. Then push X again.
Actually, there's another way to get rid of T-Rex. All you have to do is to press Y and keep running, then press Y again. Soon the monster will fall, so that you can kill it without any difficulty.