How do you kill a summoned phoenix?

I didn't know, does a hero at this level get such an ability? Well, if so, then the summoned unit is no different from the non-summoned one; the Herald doesn't care, which means the abilities are the same too.

Read about it in the fan-made manual, version 3.0. Or in some other version. It describes everything in detail. If you have Heroes of Might and Magic III version 3.0, then theoretically this manual should be in the FanDocuments folder.