Until you parry and riposte him.
Yes, a totally viable tactic. Except you never get the chance to because he swings like lightning and you're constantly trying to keep your health up? Iron Flesh you say? Sorry, you're too slow to avoid his grab which sends you flying, forcing you to waste spell-time getting back to him. Or he'll just rush you again, stab you, and deplete your health enough to force you to heal. But wait! His attack has such a range that you're 15 feet from him, midway through an estus drink, your spell has worn off, and he IMPALES you.
There is a lot that is good about this game. A whole crapton. But FROM, for some reason, though
HARD was all the players wanted, and they sure freakin' delivered on that. This is, without a doubt, that hardest game I have ever played. And that is not a good thing. There is such a thing as "too difficult", and Dark Souls crosses that line. Not just with Gwyn. Demon Ruins is the dev team giving the finger to the player. Bed of Chaos is entirely luck-based (what with those handsweeps), and the falling floors give no warning.
And then there's those myths perpetuated (by myself at times, I'll admit), that Dark Souls is always fair to you, and every death is your fault, and that once you beat a boss you have overwhelming happiness.. No, those are lies. This game is very picky. You can be standing right behind a Black Knight and it won't backstab, or the enemies attack will clip through a wall you thought you were safe behind, while you're doesn't do crap. (LOST IZALITH DINOS I'M LOOKING AT YOU! YOU, TOO, ENEMIES WITH LONG WEAPONS!)
And the boss feeling? Lie. The only feeling I get after beating a hard boss is weary elation. Elated that I can actually move on and start having FUN again. Except for Sif, there has been no boss in Dark Souls that I've actually had fun going up against. Bosses should be the capstone of enjoyment, the thing that makes or breaks a game. A good deal of the bosses in Dark Souls are, I'll admit, easy enough (read as: boring), but some are just utterly ridiculous, and make you want to rage quit. That's the worst of both worlds. Gwyn, while being overwhelmingly difficult, is actually a good capstone for this game. He perfectly encapsulates all the successes and failures the dev team made. And he has a flaming sword, and that's always cool.
I was super-excited for Dark Souls, I was expecting an utterly amazing game. What I got was a good game, maybe even a great one, but not an amazing game.
Final Score: 7/10