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