Stun is an effect that can happen on a critical hit, or when using certain active skills. Until the stunned warriors next turn, it will be easier to hit, easier to crit, and when it recovers, it will have 2 less movement and attack points for that round. For this achievement, you must defeat the warrior before it reaches its next turn.
There is, of course, a huge amount of luck here. Your odds of getting a stun increase with your odds of getting a critical hit, so increasing your warriors' accuracy can help. Skills that boost crit chance, like the "Vital" series, help, too. Heavier melee weapons seem to increase the chances of a stun as well.
Once you have stunned an enemy, have all your warriors attack it to kill it before it recovers. This is usually good strategy, as well as a way to earn the achievement. Because its defenses are lower, the stunned enemy will be easier to take down, earning you one count towards this achievement. Sometimes, however, a stunned enemy will reach its turn and recover before you can kill it.