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