Totally Accurate Battle Simulator

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!

Here's a guide on where to find the tree giant.

The Tree Giant has to be found before it can be used in Sandbox and the campaign. Go to Medieval 1 and look around on the edge of the left side of the map for a lone tree, smaller than the trees that make up the rest of the forest. Look at the tree until it starts glowing. You may need to reposition your camera to get it close enough.