From The TimeSplitters Wiki

This challenege is set in the Subway map. The player takes control of two ceiling mounted guns, placed at either end of the main tunnel. One end is on the left side of the screen, the other on the right. The left analog stick controls the left gun and the right analog stick controls the right. The player has to shoot a number of Berserker Splitter enemy waves.

You finish the challenge when both turrets are destroyed.