Pulling a "Coyle" suggestion out of the obscurity of another thread ...
Make keeping Coyle alive an optional quest objective for both missions in which he is present and be done with years and years of frustration by players over a single NPC. After converting "Coyle stays alive" to an optional objective, increase mission difficulty in those same missions in other ways. Here's one possible example, with some supporting narrative:
"Shortly after the legendary Coyle is defeated, mobs that had until then been cowering in the shadows, because of the dread they felt about facing "Coyle the Mighty", feel emboldened enough to attack the party, for the next few waves."
Translated: -- The penalty for having Coyle die would be that more mobs attack the party for the next several waves. Then players get a choice about how to finish the Coyle missions -- they can let the mobs have him and face the music, or they can try to keep him alive, to keep spawn sizes down.
There are plenty of other ways to ramp up difficulty for those missions, instead of sticking with the initial premise that the most internet-meme-worthy NPC in DDO must be saved, no matter what.