for hard: 1 shooter, 1 ammo on pistol. Shotgun/1 fixer. Earthquake. Tesla. 20 shooters/Missile. Wall. Upgrade reload on missile/shotgun. Upgrade missile count. Sell earthquake, get laser cannon. Upgrade laser cannon. Sell tesla, get more missiles (or perhaps another laser cannon, or perhaps a shield generator). Upgrade all turrets. Get shooters throughout when nothing better to do (I finished with 86). Get max walls and 30ish fixers (I finished with 40); even if you miss the big tankers, the fixers will fix you right up. Get largest gun only when turrets fully upgraded. Max its reload and ammo count. It's only better than combat shotgun for dealing with giant walkers in the last few levels (with maxed ammo and reload, which is too much money unless you have nothing better to buy). I find the first 10ish levels by far the hardest. Once the earthquake and tesla are up it is a breeze.
Maybe extend the timer on the archer bunnies? They have longer range than just about everything and it is too easy to mass them up into an unstoppable ball.
Then again, without that, game would have been much too long. Speedup button :).
If you are having trouble with the last mission, realize that recycling the level 1 miners and the energy relays is a full refund. Level 1 miners are also the most cost-efficient; upgraded miners I guess are more energy efficient, but, better to throw down another solar plant so that you can fuel the THELs later. Try to mine out the whole system as fast as possible, dedicating as little defense as you can get away with to achieve that efficiently. Finally, recycle everything but a small armed to the teeth core.
The AI will attack the nuetral units before yours. Just leave the large stacks of nuetral units alone for the enemy to wear down. In the meantime take over the little 8 places and any places your enemies leaves open, especially his big (production wise) bases. If you take over a static spot you increase your production by x. If you take over an enemies spot you increase your production by x and decrease your enemies production by x -- 2x difference. That's the key.
 "Valthirian Arc 2"){kind=icon}
 "Storm the House 3"){kind=icon}
 "Crystal Story II"){kind=icon}
 "Arcuz (Full Version)"){kind=icon}
 "JellyGo!"){kind=icon}
 "Caribbean Admiral"){kind=icon}
 "Balloon in a Wasteland"){kind=icon}
 "Paladog"){kind=icon}
 "Adventure Story"){kind=icon}
 "The Space Game"){kind=icon}
 "Typing Ninja Hunter"){kind=icon}
 "Momentum"){kind=icon}
 "Pirate Defense"){kind=icon}
 "Nano War"){kind=icon}