What you are trying to accomplish is impossible. The WUBI runs from inside Windows. It creates a
virtual partition in a big file inside
Windows partition (such as C:). The bootloader is tricked to look inside the Windows partition for the Ubuntu partition that is actually a file.
By destroy Windows 8 if you mean to reformat the Windows partition, then you will lose your Ubuntu installation as well. Without Windows and a Windows partition, there is no way to install WUBI.
Create a live USB either from within Windows by http://www.ubuntu.com/download/help/create-a-usb-stick-on-windows OR from within Ubuntu:
Boot your computer from the USB stick and install Ubuntu overwriting Windows and everything else.
Before you do this, make copies of all your personal data, music, photos, and whatever else you value. in an external hard drive.
Hope this helps