This guide tackles the achievements brought in with the Christmas Lobby.
With this lobby, there are four new achievements. To access the lobby, you can select online > host > and pick your lobby from there - you can also choose whether people can or cannot join you through the lobby menu.
Zip line down from the mountain in "Christmas" using a candy cane. Look around the lobby level and you will see a zip line travelling down to the house near spawn. There are candy canes up near the top, so you do not need to take one up with you
Shoot the Christmas angel off the tree top in "Christmas". For this achievement, you'll want to make sure the fireworks are enabled for your lobby. This is done by typing '/bang' for on or off in console commands. Once they are enabled, you want to get as close to the angel as possible for an easy hit and press 'e' on keyboard to fire the fireworks.
Top the Hat
Stand atop the large snowman's hat in "Christmas". This achievement is as simple as it sounds. If you're a veteran player, you could probably climb the snowman from the bottom with a bit of elbowgrease but there is an easier way. By re-aligning the catapult towards the snowman, a one-below-max power shot will send you flying to the top.
Deck the Halls
Land in the snow fort in "Christmas" on a snowboard. This is probably the hardest/most iffy achievement to get. You want to travel up the slope (catapult for easy access) and jump on a snowboard. From there, you want to align yourself so you do not hit the tree nor the ramp halfway up - this ensures maximum momentum, then jump as you hit the slope at the very bottom.
It should be noted that this achievement seems to be a lil' buggy, so keep trying and hopefully you will get it! If the solo attempts really are not working, do join a public lobby, it seems to work better there - if one person makes the jump the whole lobby will be credited for the achievement too.