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