Took me 4 days. Just keep upgrading. My main base buildings were up to lvl 12 or 13 I think, and my heroes were all level 28-30. Don't bother upgrading the third planet with anything but Ore, and maybe Energy. I never produced ships there or used upgraded anything there, and only a little bit on the second planet.

 The final boss has 30M army, but all missiles. I had 2 heroes with battleships and 3 with cruisers. I gave them all missile defense, armor, and attack, and a variety of weapons. My army was only 23.5M at the end by ended up winning. I was kind of surprised, but if you equip the right abilities, it helps a lot. Make sure your 5 heroes used to fight have command or war as their specialty, and I kept command and war upgrade points about even for them. Make your other 3 heroes build. skill. Tech skill isn't necessary early in the game. Make sure all your heroes are at least green.