This is a great question. Each person has strong points.
If we look at Rainsford, his strong points are his vast experience in hunting and his knowledge of the world. We know that he travelled extensively (as he is doing at the beginning of the story). So, he must have picked up various ideas and skills in his travels. That his knowledge is vast can also be seen in the books that he wrote on hunting.
If we look at General Zaroff, he also has strong points. He knows the the terrain of the island very well. This gives him a huge advantage. Second, he, too, is an experienced hunter. Third, he read Rainsford's books. This shows that he knows his "enemy." Finally, he has seen war in his life.
In view of these points, the advantage goes to Zaroff. The reader is made to think that Zaroff will destroy Rainsford.