Some formations of very mobile troops can dodge and jink so effectively that they are hard to target.
Where your detachment has the Dodge ability, you can use special orders called Dodge:
If you wish to place any detachments on Dodge orders then you must do this in the Start phase. 
Place the Special orders marker to indicate that the detachment is on Dodge orders. While Dodge orders are in effect:
Increase the Armour value of all units in the detachment by one, to a maximum of 6+.
Halve the total Firepower of the detachment.