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