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