I feel retarded, but I can't beat The End.. Can't ever find the old man.
That's the best boss battle of all time, for all games included.
The best way to find him has already been offered. Use your map and look for white circles. Those are his sniping points. Sneak up behind him if you think you know where he is. Stay out of his line of fire. It's very possible. Red circle means he's located at that sniping point.
Use all your equipment and weapons. One way is to use the fake death pill. He comes right to you. Then, use your survival pill and nail him. It's probably the "cheesy" method mentioned earlier, but it works.
Next, I'd just use the Mic and the map. Circle 360 degrees slowly and you'll hear his breathing or talking.
Next, look for his bird. It's usually around you keeping and eye on you. So is every other animal. He uses animals to see you. Kill them all and you can to better sneak up on him. Use the thermal goggles to find the animals and his bird. But, don't kill his bird; tranq it, capture it and then let it go. The bird will fly over his head screaming, "Grandpa! Grandpa!" He tells it to shut up frequently and you can easily hear him with the mic. Then, look at the map and the red circle will be the sniping point he's at.
Next, wait for it to rain, and he'll fall asleep. You can sneak up on him more easy that way. But, he can still see and hear you, just not as well.
Again, look for his scope glint in the sunlight. As a general rule, if you know where he is, you can set up mines all around any possible escape routes he might use. Be careful running after him. You don't want to run over your own mines.
Once upon him, use thermal goggles to follow his footprints and run after him. Use all these methods, and you can bag him in about 10-12 minutes. The first time it took me 45 minutes to bag him, but it was fun as hell!