This achievement is completed in the Teutonic/German campaign, mission 4
towards the start of the level, you'll come across a 'grid' area with an absurd amount of patrolling enemies, and long grass to hide in between. There are six traps spread around this grid (biased to the right hand side from memory) just have your unit attack them from cover of the closest grass.
Once you kill all 6 the achievement will pop immediately.
As will all other Ancestors Legacy achievements this can be completed on easy.