Fencer, your idea of "Would you dare to win Espionage by capturing opponent's base without revealing it by a spy?" is good but rather easy. Just capture it with a sab! Perhaps modify the achievement to "Would you dare to win Espionage by capturing the opponent's base with a 4 or 5 without revealing it with a spy?"
(hide) If you are waiting for your turn, click on "change" next to the "refresh" on the main page, then set refresh the page to 30 seconds to get your turn to move displayed faster. (Servant) (show all tips)