The techniques of the legendary Finnish sniper: Simo Häyhä.
During the Winter War (1939-1940) between Finland and Soviet Union, Simo served as a sniper against the Russian invader. Operating in temperatures between -40ºC and -20ºC, and dressed with white camouflage. Häyhä killed more than 500 enemy soldiers. The counting of the dead bodies was done by the Finn snipers themselves.
He chose not to use telescope lens to make himself a smaller target. Sometimes a sniper is spotted by the reflection of light onto his telescope lens. Also, at this extreme low temperatures, lenses could break easily.
Another trick was to use compact snow in front of him to avoid spraying snow after each shot. A tough-guy trick: filling his mouth with snow to not let vapor come out his mouth. He was short, 1,52 m, an extra help to become invisible in the combat field.
Imagine doing all this in this example video of an airsoft game on snow.