Note: This cheat requires a basic knowledge of using a zip program and a text editor. Please back up any files before changing them.
Locate the file game.pak in the installation folder. Rename this file game.zip and use your zip program (ie: Winzip) to open the file. Now extract the file\set\registry\lod.reg the the games folder with the full path. Now use a text editor (ie: Notepad) to edit the file and change the easy setting to as follows:
Now you can save the file and add it back to the game.zip archive. Save the game.zip back to the original spot and rename it game.pak. Now when you start the game set it to easy and your units will be Invulnerable.