Here is the full list of all 9 A Christmassy Christmas achievements.
You pressed the Begin button. Hurray!
Max out any meta upgrade!
Survive for the entirety of Christmas Eve!
Survive for 30 minutes in Endless Mode!
Successfully exterminate Mecha-Santa!
Collect over 100,000 Souls!
Upgrade Santa's Movement Speed until it's over 900!
"Accidentally" use Your Elves as Personal Airstrikes.
Kill The Secret Final Boss, The Frost God.