"…I started Undead, bought Halberdiers right away, and just sent them in a single column. Upgraded Armor and Speed and also Spears for a faster unit at times. I took out the Demons before even attacking anyone else. After that, I always went for the highest difficulty first. Was a very easy win this way" Yeah, that worked for me too.
Really liked it. Suggestions: Maybe a separate upgrade space for your tank between missions. Also, an overheat debuff to make it a little bit more difficult :-P.
Keep on rocking man!