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