Yikes. Well, there’s really no “good” way, and by that I mean both safe to you, and an efficient and easy way to dispatch the deer in this situation. I can tell you about anatomy a little bit. Down the back of the hind leg is the femoral artery. If you can give a quick slice, without catching a hoof, that shouldn’t take too long. Although, I’d almost recommend a long handled axe, if your sure with your swing. That might help keep your body out of hoof reach. You also have the carotid down the front of the throat, but there’s usually a lot of fatty tissue and extra skin there, so you’d have to get a long, hard “swipe,” and for that I would recommend a samurai sword. Lol. Nah, a machete I guess. A bush whacker. You really need to keep your body safe from hooves and antlers. Another thought, if the deer is on its side, and you can approach from the back, rather than hooves and belly, or tail, maybe a mallet and a spike to the back of the head/spine area. I think you could reach the brain from the ear canal or just behind the ear. Another option is tranquilizers. Using one could allow you close enough, with minimal danger to yourself, to dispatch the deer in the way you are most comfortable.