The Sitre-Achre’s main weapon is illusion. He truly has no power, for only Hashem has any power. Yet Hashem gave the Sitre-Achre an illusionary power; that is, the power of illusion, in order to enable free-will.
The main place of attack of the Sitre-Achre is in the manner of despair. For the essence of the Jew is that he never despairs of hope*, for with Hashem there is no such thing as lacking hope. For all is in His hands to do as He pleases and nothing can stop him. The essence of the Jew is to trust in the Al-mighty and to always be happy and secure that his rescue is at hand, for Hashem is with him, and nothing can stand in the way of Hashem’s salvations.
So the Sitre-Achre works against this trust and makes great illusions in front of the Jew. The Sitre-Achre shows the Jew that he, the Sitre-Achre is winning the battle. He shows the Jew how poor is the general situation of the Nation, how weak are the people in the service of Hashem, how weak is the Jew himself in all his efforts to purify and strengthen himself in the doing of good, in his self-improvement, in his motivation, etc. Anything and everything the Sitre-Achre will use against the Jew, exploiting all his areas of weakness in faith and hope.
Because hope and faith are the Jew’s weapon against all enemy. For the success of the Jew in all is dependent upon Hashem alone. Hashem alone sees through the illusions of the Sitre-Achre and knows the true strength and goodness of the Jew, knows how to judge him for merit and to bring him to succeed in all his paths. So one must trust Hashem always and implore him to reveal to him the ways to strengthen himself in faith, to be able to stand against his enemy. For if one trusts in Hashem with all his being he will surely succeed and overcome all difficulty.
*We are called עם קשה עורף, a stubborn Nation. The positive aspect of this quality is that we stubbornly stick to Hashem, no matter what. Also part of this is that the Jew doesn’t give up on himself, and sees himself as correct despite much evidence to the contrary. This is because he is connected to his deepest root which is purely good, therefore he cannot accept himself as being wrong. This stubbornness is the root of the Jew’s survival, physically and, more importantly, spiritually.
Yet, in order that the Jew live in reality, he must see that his true goodness is only at its root, at his connection to the Al-mighty. The Al-mighty is solely good and pure and when the Jew continues to draw his strength and hope from this pure source, he will correct all his traits and ways as they are manifest here in this world. He will be able to remain strong in his faith in his salvation from the goodness of Hashem even though he must admit his own errors. For he is not trusting in himself, rather in Hashem.