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You need to set your system time to December 15-31, 2011. Then, you need to create a local game with bots (you can use custom maps for this). You'll need to enter these commands once the game starts: sv_cheats 1; mp_limitteams 20; mp_teams_unbalance_limit 30; bot_zombie 1. These commands make the bots stand still and allow multiple bots on one team. Then you need to enter the command bot a bunch of times until you have plenty of players. Once that's done, you can finally enter sv_cheats 0; retry. These commands allow you to restart the game with all of the commands in place as well as reenable achievements. After you kill a few enemies, enough gifts should have dropped and you'll get the achievement. Don't forget to set the time back to the correct date afterwards otherwise you'll start getting a bunch of SSL errors.

There are also some servers that run a plugin that enables the gifts dropping at all times but I don't actively play so I can't point you to anything specific.