Guide for 8000 feet: Move up, use your full boost and switch between 20-21% resistance; when your boost is almost gone, switch to 0% resistance as fast as you can - you should fly with 200 mph; than switch between 0-1% resistance and 197-200 mph (thats the hardest part); if you are near the water, fly with 3% resistance as long as you can, when your going down, move up and down and try to small resistance parts with 1%.
PS: I think 8200 feet is the maximum. People, who have much more, cheat.
The Sky Dragon can kill the DARK GOBLINS in the Goblin Challange!
The Stop-Skill ist extremly helpful in all challanges.
Use the Shield-Skill vs. BigBang-Bombs.