Assuming the equation is as follows:
\(\displaystyle \:csc^2x-cos^2x=\frac{cos^2x+sin^4x}{sin^2x}\)
I'd suggest you not bother with turning csc into 1/sin, but instead start with the right side. Break it apart into two separate fractions, like so:
\(\displaystyle \:csc^2x-cos^2x=\frac{cos^2x}{sin^2x}+\frac{sin^4x}{sin^2x}\)
After simplifying, you'll be left with a much simpler equation you should be able to prove easily.