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