From The TimeSplitters Wiki

Shrinking From the Cold
Game	TS2
League	Amateur League
Section	Mode Madness
Mode	Shrink
Map	Siberia
Character	Lt. Frost
Enemies Encountered	Nikolai Sgt. Shivers
Rewards	Silver: Shrink Gold: Private Sand
Medal Requirements	Bronze: 10 Kills Silver: 10 Kills in 4 min 30 secs Gold: 10 Kills in 3 min

Shrinking from the Cold is a pretty straight-forward match that introduces the Arcade mode: Shrink.

Briefing

The Time Portal has caused spacetime anomalies around the Dam. People have been shrinking! Lt. Frost, Nikolai and Sgt. Shivers have spiced up guard duty by making a game out of it. Remember, you won't win if there's anyone bigger than you!

Strategy

The Minigun is by far the best weapon in this match. It's located on top of the smallest building that was initially locked in Story Mode. After that, all that is required is to search for the other enemies and dispatch of them quickly. And be sure to hit the secondary fire button to automatically spin the barrels.

Another strategy to try is to pick up the Remote Mines, they are just as effective as the Minigun but they require a different means of use. It all depends on what you're more familiar with.