Aircraft Evolution (EP)

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.

Super easy game. I got this at the start of Era 4. Best to skip a few aircraft's each time. Level up the aircraft your using and then skip 1 or 2 aircraft's when looking to upgrade. I would recommend levelling up health and fire power, ignore fuel and speed as these are really not needed.

By doing the above you will have also achieved as well.