Once you've turned art into a symbol, you cannot manipulate that art unless you double-click on it (go inside the symbol), then the eraser tool will be able to affect the art. . . But this will effect every instance of the sword symbol, so I advise making a new version of the sword symbol, before erasing anything.
Alternatively, you can break the symbol (Ctrl+B) for the frames were you want to erase the sword. Once the symbol is broken, the eraser tool will affect the art. 🙂