In Shelley's Frankenstein, the central man-to-man conflict, if you want to call it that, from chapter 17 on is Victor vs The Monster, if you will.
Once Victor chooses to destroy the female and break his agreement with the monster, the remainder of the novel amounts to a "cat and mouse" game, with the monster being the cat. From that point on, the monster tries to kill everyone who's meaningful to Victor, and Victor tries to prevent him from doing so.
Once Victor fails to stop the monster, then the story is truly reduced to man vs. man, with Victor chasing the monster across deserts and ice and everywhere else in an attempt to get revenge.
Ironically, though, this great epic chase is at least in part a figment of Victor's fertile imagination. He doesn't stand a chance, and the monster certainly knows it. Taking the opportunity to mock Victor when he gets a chance, he actually leaves clues so Victor can keep chasing him.
The chase, if you can call it that, becomes the reason for both of their existences.
I realize that the monster is not exactly a man, but since he is an intelligent being, I think we can include him in man vs. man conflict.
In that case, the major man vs. man conflict here is between Victor Frankenstein and the monster that he has created. The first main conflict is over whether Victor should create another monster (this time a female one) to keep the first monster company. The conflict between Victor and the monster over this issue leads to much of what will happen in the rest of the book.