What is the ruling on one who says "Hasbi Allahu wa Ni'mal Wakeel" (Allah is sufficient for me, and He is the best Disposer of affairs) as a supplication against his oppressor?

It is not permissible to supplicate against anyone unless they have committed an injustice, and patience and forgiveness are preferable. The saying "Hasbi Allah wa ni'ma al-wakeel" (Allah suffices me, and He is the best Disposer of affairs) is among the virtuous ذكر (remembrances/invocations), especially when experiencing something disliked or fearful.