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