Pontiff Sulyvahn is a knight wielding two swords : a magically imbued one and one with fire. He is a very aggressive and difficult opponent and expects to die at least several times before you can beat him.
All the help you can get
As with all other bosses, there are NPCs you can summon to help you. Here you can find three of them and that is a good sign he will be tough. Anri of Astora, Londor Pale Shade, and Black Hand Gotthard are located around the door leading to the boss.
You can parry all attacks but the beam and aerial ones. Although it is best to go for a build with a lot of stamina regen and keep rolling.
Weakness against thrust damage, but strong against bleed, frost and poison damage.
On phase two a clone is summoned which will make the exact same moves as the original but before him. Yet it is advised to murder him as soon as possible.
1. Going in fast and shield-less
If you feel comfortable with dodging and rolling then you can try and outmaneuver the boss with light armor and fast attacking weapons. You will need items with fire resistance and stamina regen.
The idea here is pretty simple. Since the boss chains plenty of attacks in his combos, it is nearly impossible to land a hit and guarantee a safe retreat. However, you can stay at a relatively medium to close distance and wait for the combo to stop for a moment. Go in, land a couple of hits and get back.
During the second phase, you keep your distance from the summoned replica and wait for Pontiff to make a leap attack. There is a small opening after he lands and you will be able to get close to him if you roll to avoid the AoE of the landing. As a last note, keep an eye out for the beams both of them can shoot. They are a potential instakill.
2. Using a small shield and parrying
There are plenty of attacks for you to parry here. The basic strategy is simple. Since the boss has an aggressive style you should block the first attack, and parry his second attack. Note that there may be a slight delay before the second one. Should you fail your parry you have to roll out of the way as soon as possible. Using Iron Flesh is going to make this fight a little easier as it will soften the blow of a failed parry attempt.
During the second phase you can either wield your weapon with both hands and after buffing it up burn him as much as you can, or you can stay far from both the Pontiff and his clone, and do the same as above after the knight leaps towards you.
Burning him down is a lot easier to achieve if you start attacking him as soon as he begins to transform and when he is lifting his sword to perform the summoning of the clone. Even if you don’t manage to kill him during that short window, his health will most likely be low enough to kill him with only a few more hits. Don’t rush in and take your time.
3. Greatshield of Glory for the win
Among the different strategies, you can use on this boss the easier and safest one is using a greatshield. Greatshield of Glory is especially effective here and a life saver since it can block huge amounts of damage and you can simply hide behind it. The only attacks it will not be able to absorb are the beams and the fire bursts so keep an eye for them and avoid them.
During the first phase of the battle you can get really close and wait for an opening to land one or two attacks each time, then raise your shield once more. During the second phase, you should play it safe and try to kill the clone first before turning your attention to the boss. Keep in mind that he may summon another clone if you take too long to kill him. Apart from that, this is the easiest way to defeat him, as long as you have enough strength to actually use this greatshield.
4. Using Great Chaos Fire Orb
If you have a build fitting only the most dedicated fire lovers then this fight will be nothing special for you. Having at least 28 points in Faith and 30+ points in Intelligence as well as having Pyromancy Flame upgraded enough, you will not even break a sweat here.
During both phases, you can simply kite the heck out of the boss and cast a Fire Orb every time you see an opening. Since you will have some time during the animations signaling the second phase, you can burn him down even more and need only a couple more orbs at the most.