The mirror gives you an i100 weapon. The drop rate therefore needs to be such that the work required is close to that of farming the 2 and a half weeks of soldiery needed for the i100 weathered weps or to complete your i100 relic animus. In 2-3 weeks of on and off farming I'm sure you'll see and win roll on at least 1 mirror. In the meantime you build animus for your main and farm soldiery for a wep for a 3rd class.