Where in Night does Elie indicate that the only reason he is still alive is pure luck?
This will require you to pour through the narrative in a rather intense manner. I am not sure you are going to find a quote that discusses the issue of "pure luck" as being critical to his survival. The reason being is that the nature of the narrative is so emotionally wrenching and so honest from an affective point of view that the cerebral issues of luck and calculation are not as evident. At the same time, the implication of luck being a survival tendency implies some level of contentment about such a predicament. I am not really sure that Wiesel is going to take this line of thought. For example, he actually points out to moments where critical decisions were made and "bad luck" followed. The decision for he and his father to leave the infirmary and try to run away on foot was one such instance, for the infirmary patients were saved and liberated two days later. You might want to expand the discussion a bit and incorporate what it means to experience a loss of faith. This might not be exactly where you are going with the luck concept, but in discussing how religious faith was withered during the Holocaust, the randomness of survival is something that can be quite a logical move. In this light, identifying the incident with Moshe the Beadle, the first night Eliezer arrives in the camps and sees sights that cause him to compose the poem, "Never Shall I Forget," as well as the hanging of the small child could all be instances where quotations from the text could help discuss Eliezer's repudiation of his own spiritual sense of self as being critical for his survival.
Wiesel does not approach his survival as an issue of 'pure luck' but rather mentions luck in the context of securing a place in a desirable labor camp or in lining up for soup. He mentions that 'finding the right spot in the line could mean a thicker bowl of soup - which may add a week's longevity, but this entailed rough elbowing, as well as timing.' (i.e. luck)
Hope this helps!