This has to be one of the most aggravating parts of the game. The two knights sniping at you with dragon arrows while you are trying to navigate the ledge. I didn't have too much trouble with them on the first playthrough, but in NG+ they are a real pain for my sorceror to deal with.
I read a trick online, and it seemed to work really well. It's cheesy and cowardly, but what the heck. What I did was get to the platform that has 2 bat-wing demons, and kill them. Then when you look left, there's the path going up, with the archers shooting from each side. Don't go up the path. Instead, hide behind the column on the left, and equip a longbow with poison arrows. Peek out, and snipe the archer on the right. But you have to be careful. You can only get off a few shots, and then he starts shooting back (I still got hit a few times). Go back behind the column, and wait for him to stop shooting (8 to 10 shots), then come out again and repeat. If you can position the arrow right, then eventually you'll poison him. It takes a long time for him to die, and you'll have to poison him twice.
The second archer was trickier. I did the same thing, hiding behind the same column. But the shot was harder to make. But eventually, he went down too. After that, it was just a nice gentle stroll across the rooftops of Anor Londo, to the bonfire. (First door on the LEFT not the RIGHT!)
And that's where my sunbro Solaire says, "You really are fond of chatting with me, aren't you? If I didn't know better, I'd think you had feelings for me! Oh no, dear me. Pretend you didn't hear that!"