I thought this might be interesting because, even though Surtur has the firepower, he still got beaten by Thor quite easily, despite having his minions helping him as well. Whereas Hulk basically had to BFR Fenris, who could not only eat his hits but could pierce Hulk's flesh, something that very few individuals can say. I think it depends on how well Fenris can handle Surtur's flames. If it can withstand them long enough to get near, Surtur might be in trouble.