Aircraft Evolution (UP)

The best aircraft becomes available whenever you hit Level 30, which can be done as early as the middle of Era 2. Just stick with a fully upgraded second fighter and lots of repair kits and keep replaying some of the more target-rich Era 2 levels. You'll get several hundred XP each time, and a huge bonus at every level upgrade. Save the money (you'll also get to 5,000 credits for the Rich Achievement in no time), only buy repair kits and the occasional batch of bombs. You'll be Level 30 before you know it. It takes no more than a few hours.

Then, just buy the final fighter and fully upgrade it. Play the rest of the game with it. It makes Era's 3 and 4 very easy. You can even ram enemy aircraft (useful on some later stages) and buildings. Just keep healing with the repair kits.

Now for the final mission, Level 40. Be sure to have around 40 repair kits and ideally 10 or more purple hyper-bombs. Destroy everything except the big tough walker robot boss. Once everything else has been eliminated, keep hitting the boss with the 300 credit purple mega bombs. It can absorb a tremendous amount of damage, so those are the quickest way to damage it.

And as with every other level, there might still be a straggler or two near a destroyed building. If there are, bomb them into oblivion and the Level and game will be completed.