This can be done easily on the first level in a non tutorial playthrough. I did this on Commander difficulty.
To do this you need stay concealed. Organize your troops around a group of enemies so that they're at 65%+ accuracy. On the turn you're prepared to kill the enemies make 3 soldiers go into overwatch. Take your time with this as the first mission doesn't have a time limit.
Using your last soldier throw a grenade into the enemy group. The result should be at least 1 enemy being killed immediately and the others being left with 1 hp remaining.
With the initial attack completed the enemies will run for cover but will activate your overwatch. Once the enemies are dead the achievement will unlock.