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