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