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