it is easy. . . you just need to hit the little red X unde the big guy. . . or for fun you can only go up on the first block when u come in the room but no more. then throw an egg at the still small guy and kill him instantly!

I had the same problem!
What you do is stand at the edge of platform on the opposite side of Pirana And shoot eggs at his Cork heart in the middle when both his vines are underwater!
ANd if that doesn't work KEEO TRYING! Hope u make it!