How to fight enemies spiritually?

Now here is a challenge—the next time the enemy approaches, that enemy of sadness, hold on steadily, even mechanically, to the name of Christ for one-half day—if you persistently keep the name of Christ in your mind, it may be just parrot-fashion, but if you have the will to do this, instead of days of depression, a tussle of a few hours and the enemy is routed.