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