Because 見敵必墜 is what is known as a 四字熟語, you will likely have to read it with the Chinese 音読み rather than the Japanese 訓読み。 My best guess is that it is けんてきひっつい, since 見敵必殺 is pronounced けんてきひっさつ。 It means "shoot down an enemy aircraft upon visual contact."

My Japanese is still pretty bad but for me the individual kanji are: 見ーみ 敵ーてき 必ーかなら　from 必ず 墜ーお　from 墜ちる "みてきかならお" Is most likely not the correct reading for the phrase but now that you know one of the readings for the last two kanji you can look up the others in a kanji dictionary. I would say the overall meaning is something like "visable enemies will definately fall" hopefully an actual Japanese person will answer.