It's quite easy to defeat Dino Piranha. You just have to spin into the watermelon shaped thing on the end of the Dino Piranha's tail and then run off until you can get another chance to do the same. It takes 3 hits to kill it.
That is just a random dino piranha by the way. And why do my dinosaurs always come out slim? Maybe because..*shot*
Dino piranha and related (c)