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