Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. Enter the scenario that you are having a tough time with and save. Then, go to the scenario editor and click "Test". Enter your recently saved game and save it again. It will save to the scenario folder. Load it from the editor and add new units, buildings, etc. as desired. You can even make the cheat mode available. It will not go in permanently, but when you win the campaign, will say it is cleared.