The Sanctuary Guardian was actually the first enemy players encountered in Dark Souls' Artorias of the Abyss DLC, and he set the tone for the rest of the package, providing a major challenge that was much tougher than many of the bosses in the base game. Fortunately, there was a bonfire just before his arena, so even if you died, you didn't have to go too far to try again.
He's big, strong, and very fast, with lots of large, sweeping attacks like tail swipes and lightning shockwaves. He can poison you too and is highly resistant to both lightning and magic attacks. Beating this boss on the first try is almost impossible for most players.
The Sanctuary Guardian's one weakness is bleeding and he moves so fast that it's best to attack him with quick jabs and stabs whenever you get a chance and then roll away at your earliest opportunity. Like many bosses in the game, the Guardian's tail can be chopped off, which will limit his offensive capabilities and give you a cool new weapon.