It's a fun film. It's deadpan in its style. It uses nicely done 'realistic' special effects to simulate glimpses of the trolls, but at the same time they are ridiculous creatures that look like comic illustrations from a children's book. But big and fast.
Characters keep saying 'Forget what you have read in fairy tales, this is real life'. But in fact a lot of what happens is directly taken from fairy tales. For example the hunter lures the troll by tying three billy goats gruff to a bridge. The trolls can 'smell the blood of a Christian man' (which I believe is the Norwegian equivalent of 'I smell the blood of an English man') which means atheists are safe if they keep out of line of sight.
Humour also comes from the cluelessness of the students and the shifty lying behaviour of Norwegian government officials as they try to cover up the fact that trolls are roaming about the place killing tourists. The audience I was watching with laughed a lot.
Norway, incidentally, looks a lovely place to visit. Apart from the rain. And the you-know-what.