I would not use a 25 pound bow on a big buck. Chasing it down untill it dies is not ethical. You want to shoot the most weight you can to cleanly and humanely kill the animal so it does not suffer. I believe that your bow will not penetrate enough to reach vitals from anywhere farther than 10 yds, if that, but you should not attempt it anyway because an old bruiser buck would have some tough hide to penetrate. You would not want to chance wounding such an animal. I would immadgine that there would be some weight restrictions in your area you should check this before you hunt.