No, we don't have any way to deal with a planet killer and I suspect we won't for some time. Even if we had enough Saturn V's to boost the necessary payloads, getting them ready would take a fair while since they don't use storable liquid proellants. Actually, something along the lines of the Titan III with two extra stacks of boosters, to make a "X"' configuration, is probably about the best rapid response lift capability one could put together.
Now the wizarding world and the technic muggle world might could deal with one together, but that's unlikely to happen unless the wizarding world realized that without it, extinction was too damn likely.