The next thing to be aware of before this battle begins is the order you wish for the enemy to die. Who the player kills last determines the rewards they will receive. Ornstein, if killed last yields greater rewards, but fighting him is much harder. The reason for this is, upon the death of the first boss, the second will absorb their energy, get a full heal and become very powerful. Keep this in mind during the battle. Fighting both of them at once is useless since the second one will get a full heal upon the first bosses death.
The fight itself will involve a lot of well timed dodging and small quick hits. If you are of the heavy melee variety, wear the thickest armor you have, use Havel's shield or something similar, and bring Iron Flesh. This combination should allow you to stand up to the hits, and since these bosses are mostly close range characters, there is no worry of them being out of reach.
Light melee characters are going to have to focus on timing dodges. Also it is very helpful to keep one boss behind another, or behind an object in the environment, like the pillars. Do your best to never have both bosses visible and coming after you. Constantly run around to make sure only one of them is near your character. This will give you a brief moment to land a few hits on one of them while the other is forced to play catch up.
Mages will have a hard time on this fight if they enter alone. These bosses can hit hard and one slip up can cost a mage their entire health bar. It is strongly recommended that mages enter this fight with Solaire as support. He is an amazing tank, and with the player slinging spells while he holds the enemies at bay, the fight will go by much easier.
Smough is particularly weak to fire, and Ornstein isn't much of a fan of it either. Lightning based magic will not work well here, and the various Soul Arrows can do some damage, but you will need quite a few shots to drop these bosses.
Ornstein is fast, and will dart around the arena if you are to far from his reach. He also has a lightning attack that he can send your way. Smough on the other hand is very slow, but his attacks can hit you for huge amounts of damage. If you are going in solo, it is best to deal with Ornstein first, due to his speed. It is easy to lure him away from Smough and do some damage before the bigger of the two bosses can catch up.
Ornstein, when up close relies mainly on melee attacks. These are easily dodged, or with a big enough shield can simply be blocked. Smough on the other hand is very slow. His attacks are easy to dodge away from if your a light character. Fighting him one on one is much easier than Ornstein, just play it safe. After each attack you can land a quick strike on him, as he is so slow that it takes him time to ready his next swing.
Upon the death of one boss, the other will grow stronger. Ornstein will become even harder to kill. His attacks are now infused with lightning and this can spell doom for unprepared players. Smough also gains lightning based attacks if he is the last one alive, however he is still just as slow and for the most part remains much easier to dodge. Having lightning resistant armor can help lessen the blow, should you get hit during this section of the boss fight. If you plan to fight Smough last, who is the easier option, take note that he gains a new attack. He jumps into the air and slams the ground, doing a wide AoE attack that sends lightning along the floor. When you see him lift off the ground, be sure to start running away.
If Ornstein is killed last, he will drop 1 Soul of Ornstein, 1 Leo Ring and his armor will become available at Domhnall.
If Smough is the last killed his armor will become available at Domhnall.
Alternate Strategy
if you wish to kill Smough first in order to get the soul of Ornstein it is recommended that you use pyromancy on Smough as he is weak to fire damage; additionally try to always move in a way such that Smough is between Ornstein and you as he acts as a barrier between both. However, Ornstein has two nasty ranged lightning attacks that are capable of damaging you through your shield, and one will home in on you.
Once Smough is dead, it is recommended that you keep a pillar between you and Ornstein at all times. Take note that his spear can still go through the base of the pillar (due to the game's clipping) so keep your shield up. Remain this way until he decides to jump and land cause an electric shockwave attack. Once this happens he is open for 2-3 quick attacks before he recovers from it. Once he does, go back to guarding behind the pillar. Also, keep watch for his lightning stab attack, which is unblockable and will usually kill in one hit. Rinse and repeat until defeated.
