I've been searching like crazy all over the web for this song, and there's something about it that makes it very difficult to find. I found a .mus file at http://www.squaresound.com/games/kh/arrangements.html, however I am unable to open such documents. Is there anyone who can help me? I need the score for piano. Thank you very much!

get finale notepad and it will open
heres the link just create a acc on finale and download it. its free http://www.finalemusic.com/notepad/

The 13th Battle you said? at squaresound it is a .zip file. o.ò do you not to able to open a .zip file?
http://www.squaresound.com/games/kh/arrangements/gillianrenk_kh2_13thstruggle.zip