Congrats, they are the midpoint of the game and what really separates the men from the boys. The game is both easier and harder at the same time after them and they are meant to let you know from a gameplay and story standpoint that things are coming to a head.
Here's some advice from a person who took forever to beat them too:
-Use the pillars.
-Kill Ornstein first, Smough is much easier in the second phase than Ornstein.
-Summon Brolaire. Don't you want to cooperate jollily?
-Summon ANOTHER person if you have too. This takes away a lot of the challenge though, as summoning another thinking person to help you spoils the fun of victory a bit.
-Ornstein is weak to fire, Smough to lightning. Resins and pyromancy are your friend.
-For the love of Christ take it slow. There's no rush, especially in the second phase.