It feels a little slow, but not bad! Could use some sort of health bar for the zombies, to know if the larger zombies have more health, and by how much.
I am also a sucker for upgrades in games like these XD
For the hard badge, the most useful thing for me was that I made sure I was using the golden gun at the right times. The further along the right side of the screen the turtle is flying, the faster he is moving. The golden gun launches both the turtle's speed and height dramatically, so I made sure to use this whenever the turtle slowed down to somewhere around the middle of the screen, to keep him going fast. Nukes help too, though they do not speed one up (horizontally) like the golden gun does. Just try to keep your turtle on the right side of the screen, get lucky enough to collect 1 thing of ammo, and to avoid all death, and you're done!
 "Priestess The Zombie Slayer"){kind=icon}
 "Toss the Turtle"){kind=icon}