Here is what I think Rainsford would say. 

Dear General, 

If you are reading this letter, you have "won" your game. You might even feel a sense of accomplishment, but I want you to know the facts before you boast any further. Even a little contemplation will show that you did not win. In fact, you cheated every step of the way. Consider the facts. 

First, you had the advantage. You know the land; its your island. As a military man, you know how important this point is. Second, you had a gun, whereas I only had a knife. How can you say that this is fair? You also had dogs to help you, not to mention that giant, Ivan. So, if you want to call this a game, then it is an unfair one. So, don't be too proud of your victory. In such a situation, a victory for you is actually an embarrassment. 

Second, such a game only shows one thing, your madness. You are deranged and you do not even know it. I pity you. In flashes of lucidity, you will see that you are a murderer and a monster of a man. 

From one man to another, you are neither a hunter nor an honorable human. 

Sincerely,
Rainsford