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