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