This achievement is unlocked during the 5th mission of the first Slavs campaign, "Mieszko I".
The village of Kopyto is the village furthest from your base, all the way down in the southwest corner, across the river. You will have approximately 10 minutes to capture it before the enemy army starts showing up (on normal, anyway). This can easily be done using the units hidden on the map, it shouldn't be necessary to recruit any (but it'll help). The first set of units can be found right outside the gate of your fort (you can't really miss them) while the second set can be found east of the middle village, by the road.
As soon as the village of Kopyto is captured, the achievement should pop.