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