Hey!
I found a way to make Fire Stag somewhat useful:
Fire Stag has an unlimited use of self-destruct which does good burn/pierce damage in a fixed (friendly fire) AoE.
Unfortunately it kills itself with the self-destruct (obviously). So the limitless uses are of little... use.
But if you cast Lay on Hands (Shieldbearer) or Barring Death's Door (Priest) on the Stag it will not get killed in the process, allowing it to repeat the self-destruction over and over again as long as the barring effect lasts. This can be prolonged with Salvation of Time.
All those spells can be cast on the Stag because it's a "steerable" summon which is not on autopilot and thus counts as "ally".