Here is the full list of all 18 Nimbus achievements.
Cleared the final level on easy difficulty.
Cleared the final level on normal difficulty.
Bump the meat onto the grill on World 2-4!
Collected all coins.
Cleared all levels on normal difficulty.
Cleared all levels on easy difficulty.
Cleared all levels on hard difficulty
Cleared all levels at the scrapyard, on any difficulty.
Cleared the final level on hard difficulty.
Beat 14 seconds on World 1-9.
Beat 27.5 seconds on World 2-11.
Beat 30 seconds on World 2-7.
Beat 20 seconds on World 3-2.
Beat 42 seconds on World 3-5c.
Beat 16.5 seconds on World 3-12.
Beat 20 seconds on World 4-4.
Beat 45 seconds on World 4-7.
Beat 30 seconds on World 5-2.