Totally Accurate Battle Simulator

I did this on the farmer 2 map in sandbox mode. In the tribal tab choose clubman and place atleast 100-200 of them as closely as you can for blue team and keep them fairly close to the blue line. Then go to medieval and choose catapult. Place 1 of these at the top and one at the bottom of the red line above the stone wall and keep it within like 20ft of the red line so that it can just sling the rock into the hordes of clubmen; this should work first try.

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.

This is best done on Sandbox mode, set one side to a catapult and the other to have 100s of clubbers. Start the battle and with luck the catapult will get 10 on its first attack.