It pained Jon to voice out the obvious tactic for killing dragons. After all, this was his main advantage aside from magic now. But there were scant few protections that could stop or deal away with a grown dragon, and he could not perform them with the impaired magic here or without a proper wand. Just putting this information out there, it was far too likely that it could have been used against him or his future descendants. But the risk of Daenerys swooping in and freely causing death and destruction in Winterfell while he was not here was far too big for him to take. Those thoughts only fed his fury. He showed none of it outwardly, but he hated being cornered with a burning passion.