Dark Souls

Defeat Dark Sun Gwyndolin

In order to fight this boss you first have to find the Covenant "Blade of the Darkmoon"
Gaist Heidegger has a great guide on how to accomplish that

After you get your covenant achievement walk through the fog wall and you will be treated to a cutscene, then the boss fight begins.

*note that passing through the fog will break your covenant and turn the Anor Londo firekeeper hostile, killing the boss will lock you out of the "darkmoon blade" miracle for that play-through.

Gwyndolin will be at the end of a never-ending corridor and throw 3 different random attacks your way;
he'll raise himself off the ground and fire a few volleys of arrows your way - you can raise your shield and defend or you can hide behind the many pillars along the walls,
his second attack he will raise his staff and fire a group of homing magic arrows - hide behind the pillars against this attack as it will damage you even if you're defending,
his third attack will have him raise his staff and fire a large magic attack - you must dodge this it as it will travel through the pillars.

Once you get close he will teleport down the corridor and continue attacking you so when you're a few pillars away dodge his next attack then sprint towards him and get a few hits in before he teleports. Fire seemed to work very well against him, a high level pyromancy and chaos fireball should make this fight a lot easier.

You can also beat this boss using a cheap poison arrow strategy. Equip a poison arrow and with enough range you can shoot him without him engaging the fight. Even if you do no damage, after a few arrows (sometimes up to 8) he will still be poisoned. The poisoning does a little more than 400 points of damage per poisoning, so after a few sessions he should be dead without the need to get your hands dirty.

Also, killing this boss makes you permanently sinned in the game and Blades of Darkmoon will always be able to invade your game (even if you request absolution of Oswald).

If you're having trouble with a melee approach just follow 3 rules.

1) When he casts the homing soulmass (lots of small projectiles) hide behind a pillar

2) When he casts his big single projectile spell dodge (roll) diagonally sideways (or roll straight backwards if you're behind a pillar, as it will go through the pillar)

3) When he raises for his arrow attack zig-zag run towards him. Make it a fairly wide zig-zag and his arrows will all miss you. Eventually you'll get in close and you can hack him a few times. When he warps attack the empty space because he still takes damage when not visible for a second or so.

I recommend doing this as the last thing you do before the final boss. So you don't lose the use of the useful anor londo bonfire (which you will if you even enter the fog gate)

just adding to the above solutions make sure that u kill him on your first playthrough because he is seriously a 1000 times easier than your second or more playthrough. killed me in about 3 hits and im lvl 192

For New Game+, quite simply.. Dragonslayer Greatbow
In the large hall before Ornstein and Smough.. theres 2 giants at the bottom, and a Black Knight with a Dragonslayer Greatbow on one of the stair ways.. at the top is a stained glass window, and it's broken, slip through there, following it around you'll find the bow.
The Dragonslayer Greatbow needs 20 in each Str & Dex. The Giant Blacksmith can upgrade it for you with Twinkling Titanite, and he sells the arrows for it too, 500 souls each (which is why I mentioned NG+), there's also the Hawk Ring behind the Giant Blacksmith to improve the range for better damage. On NG++, I had 50 , 25 would do the job with 55~ Dex

In case you don't already know. To get to Dark Sun Gwyndolin, you need to rotate the staircase to the lowest position, and go down the stairs to a bonfire. Equip the Darkmoon Séance Ring from the Catacombs.

For the actual "fight", After the cutscene, I waited for him to teleport away, then moved forward a few feet, use the walls/pillars as a guide. you don't want to aggro him. aim the Dragonslayer Bow just above his head for the headshot. I was hitting for around 100 damage on average, as the bow seems to knock him back, I didn't advance incase that aggro'd him. He has around 10k HP on NG+, no doubt more on NG++