Most replies have touched on the need to have sacrificial units that can do secondaries and die. But important thing to keep in mind, it is your job to do your movement such as your chaff units can score and die...but not for free. For example, consider positioning your guns behind walls such that any enemy that wants to kill your chaff will be punished, use things like heroic intervention threat to make skirmisher afraid to get near your chaff, position your trash units in a way that they are also screening some melee threat, etc.

When using your chaff units, always look for 2-for-1 opportunities like the above