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