Returning the letters
ליל שימורים הוא להשם
Amalek is determined to destroy klal yisrael. One of his primary weapons is to contaminate their personal holiness, the os bris kodesh. אשר קרך בדרך, from the same root as קרי, as Rashi says on that pasuk.
Hashem says אין השם שלם ואין הכסא שלם, His Name and Throne are incomplete until Amalek is eradicated from the world. The Name is missing the letters vuv and hay. The throne is missing the letter alef.
ליל שימורים refers to one who guards his holiness (the primary time of temptation is at night). This will cause "הו"א להשם", the letters hay vuv alef will be returned to the Name of Hashem and to His throne.
Likutei Imrai Chaim