Here's a video guide, but let me walk through it if you want it word by word.
1. You will need to unlock the tree giant by finding him on the Medieval 1 map. It'll be essentially a tree near the edge of the map.
2. Pick any sandbox level.
3. Go to the category "secret units"
4. Pick the tree giant, that is on the third page, near the far right.
5. Put him against a lot of halflings. I did around 100 to be safe, more will start lagging the game.
6. Start the fight - and the achievement is yours!