i got 3+ stars on the following levels: 1-2, 1-3, 1-6, 1-12, 1-20 and 2-1, 2-3, 2-4, 2-8, 2-9, 2-10, 2-11, 2-12, 2-13, 2-17, 2-18, 2-20
i got the badge without even playing the last 2 levels.
for some weird reason i don't understand, the hard beadge gets easier when listening to rock music. Don't ask me why, but it works, see it for yourself
How i got the hard badge: In the first day, max out the engine, transnmittors and the wheel. Use the remaining money on the fuel. You should earn about $6800, so in the second day buy and upgrade the weapon to the max possible. Use the remaining money to max out fuel. If you are lucky, you can earn even 11000, but you're probably gonna get 7500-8000. To the last day, buy the propeller and max it out, if possible. Use it to fly past the crates and get your badge!
a hint for the hard badge: after you beat every level, you get something called jukebox, that allows you to choose the level song. that way you can make the level easier to beat. I personally used the music 'the terminal', (the piano one)
the computer i was playing this had no sound, so i gave this game 3 stars. then i played it again on my computer, with headphones, and just had to give it a five.
some hints the hard badge: first of all, if u can play it on a slower computer, do it. a little lag when u are going up is very useful. second, try to get high combos when stacking the cakes, it gives you some bonus poins to start. third, be kind and thumb this up so others can see