To be honest I don't see why some people whine about this boss fight. The boss is not hard. The boss is hard IF AND ONLY IF you are going for the Moonlight Greatsword. In other words, getting the Moonlight Greatsword is hard. That is the true boss fight: the struggle to catch Seath's elusive tail.
Otherwise, there are many ways to cheese him. Cheesing while circling, as the page recommends, works. You can also cheese from quite long range as he creeps at snails pace towards you, and then, when he finally gets there and starts attacking, bolt around him to the other end of the arena and cheese more. It's really easy. You can lock on from the other side of the arena and use any ranged attack you'd like. I figured it would be fun to use the Avelyn I just acquired in his Archives.
But getting the Moonlight Sword...good gosh that is hard. For one thing, you have to hit the last three yards of the tail (give or take). That little tail wiggles like crazy and sticks itself behind walls constantly. And if you manage to get behind Seath in a good position, he pretty much always will try to do his melee-slam attack, which you CANNOT block. Even if you block one, multiple other hits are on the way, and they will hit you even while you are on the ground (most attacks can't do that). Turtling is a non-option. Your only chance is to run far away from him. So maybe 1/4 of runs for the tail will result in a hit on it. And you really would not be wise to try to hit the tail multiple times unless you have a fast weapon. Greed is punished severely by his melee-slam.
TIP: do not let yourself get squeezed into a tight space between him and a wall. If he does the slam attack, and you can't get away, you're toast, even if you are level 80. When it comes time get behind him, circle around whichever side is farthest from a wall! This means if you might start circling, then fail, then you will need to turn around and run in the other direction, running right across him in front of his face!
Surprisingly, he will let you do this.