Here is my 100% Achievement Guide!
This is for defeating the final boss.
He has 3 forms.
The first is simple, you just have to keep climbing up the platforms as he attacks you with his bite..
To attack him you need to grab the black jelly baby looking things, use the mushrooms yo jump high and throw it at him.. do this 3 times..
Next form, (gave me the most trouble)..
You have to do the same thing, on platforms, grab jelly, throw it. Careful though, remember to JUMP rather than walk, as there are gaps in between each platform so you fall to your death.
His attack this time consists of shooting a thing with 4 arrows sticking out of it which shoot in random directions, so pay close attention to that when it arrives.
Final part is easier thankfully, we keep going to the right, looking for jelly and mushrooms to hit with, itherwise this time he uses his hands to destroy the platforms your on, but he can't reach the sort of middle bit where your on, so long as you stay in that middle bit, you should be fine!
Go to 54:05 in my video to see exactly where this is done
Hope This helps!