This achievement; http://imgur.com/RGHKOIz (Cold Hearted!: On Deathrun, kill a Snowman whilst a Snowman yourself.) Isn't obtained when you actually do what it says. We have 6 people on the server so I'm not sure why it isn't registering.
This achievement doesn't actually require 6 people - only Winters End does. I've just tried this locally, and it seems to work find for me. You were both Snowmen at the time? Who killed who? A runner killing a death or vice versa? And they did physically kill them? Do you get the death Notification in the top right saying Player X killed Player Y?
I just got that achievement and so did others. We all got it after we beat the map as a runner and killed the death with a gun.