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