No it's just easy to understand when something can't be killed by mortal weapons, but can be harmed by them. I mean the first time you said he can't be killed by mortal weapons I understood what you meant.
All I really needed to see was "Grima can't be killed by mortal weapons."
I think if I did, then I was mentioning the overall dragon. The nape of his neck, where Grima's Avatar is when he's with the Dragon, is the main weak point for mortal weapons to harm him. And if you want to know how to go about getting that information in the IC, the solution is unconscious in the medical tent on the ground.
in other words, Grima's Avatar is his weakness.
EDIT:
Oh and before I continue, I really need to know if Air Support is there. If it is, it would be relatively easy to drive him off considering the fire power. Just need to concentrate fire on his neck.