In the rain, where I especially like to hunt, I cover up well with my rain gear, keep the rifle out of the weather, and expose the rig only when it is time to shoot. Water beads collecting on the lenses will make a shot impossible due to distortion and image abberation. Keep a soft cloth with you that is dry then wipe the lenses clear. If it is raining so hard that you can't get the lenses dry, your shot is presumably close at hand, in this case look down the barrel of the rifle; forget the scope.
I have heard, but not used, Rain-X will keep a lens from fogging or collecting rain drops. I have ridden in one automobile following a windshield Rain-X treatment; it worked very well. No need for wipers and none were used.
I doubt that I will try Rain-X on the expensive and coated lenses of my rifle scope. I am not about to ruin an investment because I prefer to hunt when it is wet.