How could Obama intervene in Syria against PISSI.

There is no obvious feasible way for the US to fight PISSI in Syria without helping Assad. The non-jihadi Arab opposition to Assad is almost nonexistent, and supporting it would be meaningless.

The only other forces in Syria opposed to Assad and PISSI are the democratic-socialist Kurds. I'd love to see them get support, but they may not want to do more than defend their own territory, and they may not be welcomed by Arabs.