Examples of "economic obsolescence" include airport noise, toxic waste, nuclear power plants, freeway noise, dust and air pollens, changes in zoning, and more. In your case, it's the apartment building across the street.
Properties such as yours that are faced with "economic obsolescence" will experience reductions in value. As such, given that you already had 18 showings in a month, you should consider reducing the price to help induce someone to overlook the issue and buy your home.
The only other thing I can suggest is point out positives to the neighborhood that would not be available in other neighborhoods...... walk to train, walk to park, walk to downtown.... whatever it may be.
Sorry, I wish we had a better answer but unless you can convince the owners of the place across the street to fix up their home to benefit the neighborhood you are going to have to get more creative.