Here is the full list of all 20 Really Big Sky achievements.
GET SOME KILLS
BE RISKY
NUKE ITS BROTHER
GET A LOT OF KILLS
USE THE BOOMER
CLOSE ENCOUNTER
FIGHT BOSSES
GET MULTIS
LAST A WHILE
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