Frankenstein is a fantastic tale that is, notwithstanding, told by an objective reporter, Walton, who is introduced in the beginning of Shelley's novel. This frame story technique is certainly effective in lending credibility to the narrative. And, with the epistolary form in which Walton writes back and forth to his sister, a form which frames both Victor's tale and the creature's history, a sense of realism is created.
So, in writing your introduction, you may wish to point to the objective underpinnings that the narrative has which lend verisimilitude to Shelley's fantastic tale. Your thesis can make a statement about this technique and how it is effective. Then you can demonstrate in the body of your essay how Shelley's technique lends a certain credibility to the narrative.