From The TimeSplitters Wiki

 Your objective is to hold off the zombies until time runs out.  In the beggining, zombies will spawn from the open door leading to the outside.  Take careful head shots until you either complete the wave or the sewer zombies get to close.  Then retreat to the back of the building.  Sewer Zombies will now spawn from withen the building as well.  Try the "double shot method" for close combat.  Keep moving at all times.  Try to group your enemies in tight balls, this way the ones at the back will hit the ones in the front thus killing many zombies at once.
 Remember, zombies are melee attackers and head shots may not kill them.  Complete waves to gain bonuses.  Avoiding being hit will also give you more points.  Zombies are slow but have high stamina.  Keep this in mind.  Grab ammo when available but do not risk your life for it either.[[1]]