Here is the full list of all 20 Really Big Sky achievements.
LOTS AND LOTS OF SMALL THINGS
GET 10 LOTS OF HIGH SCORES
DIGGING HELPS
BE RICH AND THEN NOT RICH
GET A LOT OF KILLS
DON'T KILL DEAD THINGS
NOT TOO IMPOSSIBLE
USE THE BOOMER
FIGHT BOSSES
KILL BIGGUNS
REMOVE BOSSES
GET MULTIS
CLOSE ENCOUNTER
LAST A WHILE
BE RISKY
GET SOME KILLS
NUKE ITS BROTHER
GIVE A HOOT
HAVE SKILLFUL BUDDIES
BE LIKE HARRY STAMPER