How do you kill a summoned phoenix?

On a global scale, it might be more, but if players, with standard upgrades, are having problems specifically with the Phoenix, then the standards need to be changed.

Well, the Paladin-Angel-Marksman combination under General Blessing or punishing against the Phoenix is very effective. :p You can also subdue the Phoenix, or Berserk it, or Blind it. These are also options. ;)