Mikhail Lermontov's narrative poem The Demon is a text that obsesses with images of isolation and alienation. In fact, the opening lines set the tone for the rest of the poem:
"A Demon, soul of all the banished/ sadly above the sinful world/ floated, and thoughts of days now vanished/ before him crowdingly unfurled."
Readers are immediately presented with the image of the titular Demon separated from the world. He is exiled, banished from the world of the living. The image is especially potent because he has memories of better times, and this exacerbates his sense of isolation. The Demon has not always been this alienated from the world; instead, he suffers even more because he can remember sensations of a past life. Even when he successfully courts Tamara and kisses her, his kiss kills her, and he is again left alone, unable to connect with the world below.
As far as essay tips, there are a number of both subtle and blatant moments in the text in which the Demon alludes to his isolation. Also, a number of critics apply biographical readings to the poem. Lermontov wrote The Demon while he was in exile in the Caucasus. The fact that the poem is set in the Caucasus, and Lermontov's continual emphasis on the Demon's isolation, lend credence to this reading.
Here's an interesting source that may be useful:
Golstan, Vladimir. Lermontov's Narratives of Heroism. Evanston, IL: Northwester UP, 1998.
I used Charles Johnston's translation for textual support.