I can't tell what is wrong with your lilac, based on the photos.  It could be verticillium wilt.  Or lilac borer. Or some other insect or disease.  Below are some references. 

I would definitely not cut the lilac to 12 inches.  Now is not a good time to do that kind of pruning, and it wouldn't be a good idea to do such a drastic prune to an ailing plant at any time. You can prune out any dead branches, however.

Here is a good diagnostic site (you will see in this site that a couple of low resolution photos won't help a diagnosis):
https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/

Second, some information about a mysterious lilac disorder two years ago:
https://extension.umn.edu/yard-and-garden-news/lilac-issues-season

Finally, here is a link to the plant disease clinic, where you can send a sample for testing.
https://pdc.umn.edu/submit-sample