FYI: Here was my solution when I was a student asked this question.
In my scenario I got behind the mirror and pointed the front down the revine at the trolls and said "Mirror, show me the mirianis trench 5 meters above the bottom of the deepest part." The marianis trench is miles deep so the water shot out at the trolls like a hundred firehoses and the flood pushed them back down the mountain. I said "Mirror, show yourself as a normal mirror" and it did and the water stopped spraying. I walked safely away with the mirror.
After homeworks have been turned in I'll tell you how an associate used the mirror. Looking forward to seeing your ideas, R Ravenclaw.