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