If the DM doesnt want "his character" to die, he/she wont. Sorry, but thats the way it is. Maybe this npc is important to the plot, or in some other way significant, or maybe the DM thinks you like fighting this person, but i dont know.
It sounds like the DM is being a little unfair in this situation (the use of divine intervention to save an important npc sounds heavy handed, and since true resurection can bring back someone who has been killed by a destruction, so maybe he's trying to send you a message about the futility of fighting this guy right now) but i dont have the whole story and was never present at your game table, so what do i know.
Your best bet would normaly be to kill everyone that knows his name, but then you would still have that god running around so why bother at all. Maybe you should consider joining him?