Warlord, so you need to reach Information Era to have United Nations, in a peaceful game. With only one civ alive, you just need to reach Atomic Era. You can clear the map from start to end (chariots archers rocks on great plains). Tommy made it in a GOTM with Venice (74, I think). He gifts cities to the same nation. So at last, AI razes cities for you. No unhappiness, and some gold sometimes. I start to do this, gifting cities to an AI lost in forest. Issue are barbs. Because there's a lot of free space after you burn all people, barbs spawn again and again. They come with same techs, so you manage barbs infantries around T180-200. On great plains, that's a lot of barbs camps. Without mountain, peaceful, you can't make a better time than with war. It's just a big lot of additionnal clicks.