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