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