Use the following trick to defeat Megalith with an "S" rank. First, destroy all Yellow aircraft possible. Next, fly past Megalith until you see an red-orange triangle. Shoot that aircraft down and return to Megalith. Go through the right tunnel and shoot the generator. Continue to destroy the next generator in the left tunnel. Now, go to the side of Megalith and destroy the last generator. After all generators are gone, destroy the two missiles that appear just ahead. Turn around as fast as possible, as the next two missiles will only remain for a minute after the generators are destroyed. Next, destroy the big missile in the center and do not forget to pull up.