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