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