I did this in the last level (the sanctuary). If you follow the regular path you will eventually get to a garden with a shrine on the right. This is right before you enter the last big outside area with a bunch of enemies. In this small garden is a broom on the ground near the shrine. Take it to the last area, get the attention of all enemies in there and run back down the stairs and to the right. There is a small area with a statue and an npc. Try to wait for a moment where at least 3 of the enemies are close together, run in and use the 360 focus attack (the one that needs a staff). When you hit 3 of them the achievement will pop after a few seconds.