This achievement is most easily done on 20BALLRO.lvl
From the start of the level, jump down, follow the 2 enemies to the left and throw your katana using
. The katana must go through the first enemy and also kill the 2nd one. It may take some trial and error, and the katana is a bit unpredictable after passing through the first person.
The trajectory of the katana is straighter if it passes through less of the enemy's body.