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?"
SL-Mark: it depends on opponent, if your opponent is a weak player, then you can have a great excess in pieces, so you can capture his base with anything you want
trohat: Correct, I had thought that through, for some it will be impossible, some luck, some easy. For the latter, it becomes a 50:50 probability. (edit: actually it's a probability of 1 if it is a 4 or 5. That's if you haven't resigned by then! )