Earning achievements.

I don't remember this map very well. Perhaps you were besieging a fortress with a hero inside that you didn't notice? If not, try simply replaying the chapter, ignoring secondary objectives and focusing on this one. The achievement can definitely be obtained in this version of the game.

Added 3 minutes later
Although... sometimes it happens that a certain achievement is awarded upon completing a chapter. But again, I can't remember exactly how I got it.