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