Hitman 2 Holiday Hoarders - Video Guide

To unlock Santa 47 outfit you need to complete 4 challenges: Santa's Little Helper, Oh Deer, Ho Ho Ho and Bad 47. It's possible to do it in 2 runs (combine first 3 ones and then do Bad 47 on second run).

Collect 12 items before Smokey and Slick manage to do it. You can also combine this challenge with Oh Deer and Ho Ho Ho.

I recommend doing them with Santa's Little Helper challenge, so you won't waste your time and wait too long.
First of all, you need to find Santa and get his outfit. If their mission had failed they will show near the garden. There you can kill them with a propane explosion (Oh Deer) and it counts as an accident kill (Ho Ho Ho while with Santa suit).

Last and easy challenge - pacify Marv (Slick) 3 times with a brick.