There are 10 secret achievements. The ninth is received automatically, and the tenth is for getting all of the other achievements. The other eight are obtained by playing the game "incorrectly", and all can be gotten in easy puzzles (one requires being on easy). If you're stuck on one, just think about the name of the award, open an easy puzzle, and try doing what seems right. They're not too hard to figure out, and you'll probably get a bunch of them accidentally.
Or, you can just look at the solutions in the comment below.
If you are trying to get the 50x hit Combo, go for Level 12. Drive slow and accelerate only when getting the Stealth and Rampage powerups scattered throughout the middle of the road.
I think I might possibly be the only person here on Kongregate who got the hard badge legitly, without the use of any word lists or comments listing answers. This is a word game, and it's meant to be difficult. What is the purpose of playing the game if you are just typing in words someone else has listed, when you don't even know what they mean? If you want to earn it genuinely, prefixes and suffixes are the way to go.
 "Vexation"){kind=icon}
 "Tower of Heaven"){kind=icon}
 "Flash Minesweeper"){kind=icon}
 "Magic Factory"){kind=icon}
 "The Bee Way"){kind=icon}
 "Neon Race 2"){kind=icon}
 "You Have No Legs"){kind=icon}
 "Air Transporter"){kind=icon}
 "Huje Tower"){kind=icon}
 "Defender's Quest (lengthy!) Demo"){kind=icon}
 "Vehicles 2"){kind=icon}
 "Pupzzle"){kind=icon}
 "Eddie's Lament"){kind=icon}
 "Sugar Rush"){kind=icon}
 "Blue Blox 2"){kind=icon}
 "Proke"){kind=icon}
 "Just Another Shadow"){kind=icon}
 "Icycle"){kind=icon}
 "Brutal"){kind=icon}
 "Me and My Dinosaur"){kind=icon}
There are 10 secret achievements. The ninth is received automatically, and the tenth is for getting all of the other achievements. The other eight are obtained by playing the game "incorrectly", and all can be gotten in easy puzzles (one requires being on easy). If you're stuck on one, just think about the name of the award, open an easy puzzle, and try doing what seems right. They're not too hard to figure out, and you'll probably get a bunch of them accidentally. Or, you can just look at the solutions in the comment below.