Here is a video on how to do this, and my written guide:
1. Start up the campaign section.
2. Go to the "Adventure" missions.
3. Select "The Smallest Army"
4. Put the catapult in the back middle of the map.
The cannon will move up automatically as the army bunches up on the bridge. It will hit 10 people, getting the achievement.
*Alternatively, you could re-create this video in Sandbox mode on the "Farmer 1" map with halflings.