CastleStorm

There are two ways to earn this achievement. I earned this achievement easily while progressing through the story campaign.

Method 1
Like lionpants mentioned, you can earn this achievement by deflecting an enemy projectile with your own projectile in such a way that it lands on and kills an enemy. This may require good timing or luck, but it seems the frenzy level is a good level to try. Refer to lionpants' solution for details on this.

Method 2 - my preferred solution
You can use the 'Projectile Shield' spell to deflect enemy projectiles back on to the enemy troops. This is the method I used and it may be more reliable. When you place the shield it will deflect all enemy projectiles from their ballista and ranged troops. It will help if you upgrade the 'Projectile Shield' spell so the spell duration is longer. I managed to get it with a level 4 or 5 projectile shield and deflecting an enemy cannoneer shot.

Note: If you need extra cash to upgrade anything, load up the Frenzy Battle level on Ore Mine or the counterpart in the Vikingland campaign. Use the tri stone or ice projectile and mash the A button until you destroy the enemy castle. Also be sure your castle is equipped with the treasury and prison. You'll get around 5000 gold in under 30 seconds. Repeat as needed.

- Load up Kingdom Quest campaign.
- Choose the level Ore Mine - Frenzy Battle!
- Use the tri-shot stone ammunition.
- When the level starts, aim your shots so that if you split the stones right after shooting, the bottom fragment lines up with the enemy's ballista.
- After a few seconds a Knight will spawn and head towards your castle.
- Start shooting tri-stones every 1 second. (Press A, instantly press A again to split. Wait 1 second, repeat.)
- With some patience and some retries, your shots should bounce the enemy's right into the knight.

Note: The reason you need to delay shots is simple. If too many stones are on the screen at the same time, the richotted shots will immediately despawn.