On Winter Contingency a Skirmisher on the roof tops near the beginning of the level, the building you need to Sprint towards. When approaching the building go to the right, there must be two players to jump onto the roof. One player must get as close to the ledge of the roof while jumping repeatedly, another must sprint and jump off the cliff and will spawn on top of the building. On top of the roof there might be a small Skirmisher Champion and will not harm you.

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Screenshot of the Skirmisher