Yes, Phoenix pyre deal physical damage instead of fire damage, so maybe heavy armor will protect against it.
And what you have meet is not the worst--in a multiplayer game i use about 50 mages in a communion, and one of the master cast phoenix pyre, then some devils manage to reach one of my mage......
To deal with such condition, if you don't have too many mages, seperate them so that the explosion of one cannot affect others. If you use dozens of mages, then put them in rear, and use your army to block every way to them. If you still feel unsafe, then take a staff of storm
But the ultimate way your enemy can use on your pheonix power is spells like rain of stone or earthquake, which will ignore all your previous effort. Yet you too have a ultimate solution:fog warrior and/or army of gold so that one explosion nolonger fatal.
As for "not cast", perhaps if you script 5 spell, they will be too exhausted to cast phoenix pyre, but i wont count on it