Like Lynn, I envisioned these magic users to use something of themselves in their spellcasting, be it energy from the caster's body or drawn from some "other" realm through the caster's body, thereby stressing that body. Like an athlete, the magic user could recover with time and care. Of course, circumstances in this case preclude any real recovery time.
In this story, I pictured the wizard having already spent much of his/her energy in recent battles, and the final spell requires all that he or she has left to give. It's really a noble sacrifice, assuming the spellcaster is actually trying to protect the innocent against a marauding demon dragon and not fighting off a hero dragon. (Wouldn't that be a twist?)
Here's a thought. This is the opening scene - maybe the prologue - of a much longer tale wherein the student takes over and struggles to maintain the master's reputation and quality of protection for the villagers. The balance of the tale has the student working, learning, forming alliances, uncovering and defeating traitors in the village, until he or she is faced with the same no-win Kobayashi Maru scenario.
Then just leave it there and let the reader imagine the actual outcome.
