Whilst fighting a dragon, the party cleric takes the AoO and casts Stoneshape in order to create a collar around the dragon's neck attached the the floor. I asked for an attack roll to grab the dragon's neck and when it succeeded the other party took whacks at the dragon's backside. The dragon got its flatfooted AC since it couldn't move, but it's a dragon, that's the same as its regular AC. The dragon made strength checks to break the stone, but there's no break DC listed for stone and the stone was very thick around the dragon's neck. Finally, the cleric's turn again, I allow him to coup de gras since the head is stuck in one place and I considered it to be bound. Cleric slays dragon.
How would you guys rule in this situation? A reflex save to avoid the stone noose instead of an attack roll on the player's part? A simpler escape for the dragon? Or would you have allowed the CDG at all?