This achievement has
supposedly been fixed. It now correctly unlocks after obtaining all 30 stars in the Skill Streak story. However, some are still reporting unlocking it early. This video demonstrates it and may help anyone who's struggling:
NOTE ABOUT CO OP: Co op does not make this any easier unfortunately...
All credit goes to MotoGamesTV
Here are the point values required for each: (Good suggestion, thanks KinectKid333)
Challenge 1: 320,000 points (64,000 X 5.0)
Challenge 2: 285,000 points (57,000 X 5.0)
Challenge 3: 45 Near misses
Challenge 4: 255,000 points (51,000 X 5.0)
Challenge 5: 50 Air skills
Challenge 6: 235,000 points (47,000 X 5.0)
Challenge 7: 180,000 points (36,000 X 5.0)
Challenge 8: 200,000 points (40,000 X 5.0)
Challenge 9: 255,000 points (51,000 X 5.0)
Challenge 10: 265,000 points (53,000 X 5.0)
UPDATE: This appears to unlock when you have 30 stars TOTAL, across all stories.
Supposedly unlocked for getting all 30 stars in the Skill Streak story, but I unlocked this and the 30 stars for Ishi's taxis achievements despite having never played Skill Streak, and only around 20 stars on taxis. So these achievements must be bugged.
EDIT: Looks like everyone has had this happen. Free achievement, for now...