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