Hey Guys,
just won this today. This game is really easy. but I did not earn this thru my first play through. The reason is I played all the way thru on easy and I didn't use good strategy.
So with that said I recommend the following tips:
1. Play through on normal as much as you can. switch to easy when normal becomes too tough. The higher difficulties earn more score.
2. Keep your cities alive ( buy defense and attack for your cities and population)
when your cities get destroyed, A permanent barrier of lighting is there. it will fade out so you can pass through safely. but it will take a life if you pass through when it is not faded.
You can buy lighting timing upgrade to make it stay faded out longer. My second playthrough, I was able to get this and I was not even close to wave 40 and I still had all my cities. I suspect you also get a little bonus for keeping cities
3. Max out rage as soon as you can so you can dispatch enemies quicker. Save rage for the boss on boss waves.
4. increase gun power and fire rate at least once or twice.
5. Max out smart nukes early
6. Have at least 3 men or whatever you are comfortable with.
When you hit 500,000 points. this will pop.
enjoy and God Bless You!