When you shoot it in enough small pieces (pink ones) they all turn homing towards you. That's when you should do your spin/jump/squirt move.(Rotate+A+R).
Anywaz, it's quite tough, so I recommend that you stand on the seashore because they don't go into the water as easily as on the ground. Rather that than try to run for health coins in the huts, which will more like cost you more life that gained.
Hope this can get you further.