In fact if the tank starts on a black square, you must merely bomb that ONE square before all of its neighbours. You say that it's necessary to bomb all the black squares first, use that claim as proof for a lower bound, then admit in the next paragraph that in fact bombing all the black squares first is unnecessary. So I still don't see how you've shown that it's impossible to destroy the tank in fewer shots. Perhaps by some clever interleaving of black and white squares you might save a missile or two?if the tank starts on a black square, you must bomb all black squares and then all white squares. If the tank starts in a white square, you must bomb all white squares and then all black squares and then all white squares. Therefore the bombing run must be W-B-W.