Soul of Cinder
You now have everything you need to finish your SL1 playthrough. To access the fight against Soul of Cinder, you must defeat the Twin Princes.
Twin Princes
SL1 Twin Princes, Dark Souls III
An RNG heavy fight because the boss constantly teleports to a different location. You cannot predict the destination of the teleportation. Be ready to dodge any attacks that occur after a teleport. Phase one is you fighting Lorian alone. Always position your character by the left leg of Lorian and be prepared to dodge at most two swings of his sword. Dodge an attack or two and get in one hit. A safe window for attacking is after dodging an attack and your character is directly behind Lorian. During phase two, Lothric is carried on the back of Lorian, but Lorian deals the majority of the attacks. You must kill Lothric in order for both him and Lorian to die. Dodge Lorian as per phase one and try to position your character directly behind Lorian in order for your attacks to hit Lothric. Being to the side of Lorian is OK, but ensure that your attacks also hit Lothric.
Defeat the Twin Princes, warp back to Firelink Shrine, and place the cinders of the four lords on their respective thrones. The Shrine Handmaid now sells a Titanite Slab for 30,000 souls. Use that slab to fully upgrade any weapon you need. Sit at the shrine bonfire and be transported to the flameless version of Firelink Shrine. Make your way outside to find a bonfire, which is used to warp to the Kiln of the First Flame. Light the Kiln of the First Flame bonfire and you can now access the fight against Soul of Cinder.
Soul of Cinder
SL1 Soul of Cinder, Dark Souls III
The most difficult boss in the base game in terms of the huge variety of his move set. You need to practice this fight and familiarize yourself with the boss' attack patterns. Equip these rings so you can wear armour pieces having high defense while maintaining your weight ratio below 70%: Havel's Ring, Prisoner's Chain, and Ring of Favor. Wearing an armour set having high defense and casting Tears of Denial, you should be able to tank at least two hits and stay alive long enough to learn the boss' attack patterns.
In phase one, the boss can transition between any of these fighting styles: straight sword, curved sword plus pyromancer, sorcerer, and spear. In any of the melee styles, the boss has a chain of at most three attacks. Expect any attack to be a three-hit combo. A safe window to get in a hit is when he does a delayed overhead attack with his straight sword. Run or strafe behind him and get in one hit. The spear style can be tricky due to the reach of the boss' spear and the delayed horizontal double spin. You only have to roll once to get out of the range of the delayed double spin. Wait for the boss to be close to you and roll into his left arm. This might sound complicated, but with practice you can easily roll through the delayed double spin. The curved sword plus pyromancer style is the most dangerous during phase one. Try to be far away from the boss whenever he attacks with his curved sword and be prepared to dodge fire balls. A safe window to attack is when he starts to cast Power Within. You have a second to run to him and get in one hit. The sorcerer style can be the easiest to punish. Constantly strafe to your right to avoid the Homing Soulmass and roll diagonally through the Farron Darts. The Soul Greatsword should be rolled through diagonally to your right to position your character by or behind the boss' left arm. The safest window to attack is when the boss charges up his Soul Stream. Strafe or run behind him and get in two or three hits.
Phase two can be the easiest of both phases of the fight. The boss now fights with a great sword and casts lightning miracles. You have at least four windows to safely get in a hit. The first window is when the boss throws a lightning spear. Roll towards the boss and get in one hit. The second window is when he impales the ground with a lightning stake. Again, roll towards the boss and get in a hit. The third window is when he charges the rain of lightning. Close the distance as quickly as possible and get in two or three hits, then move away and be ready to perform three rolls to dodge the rain of lightning. Finally, the fourth window is when he two-hands his great sword, performs a combo with insane range, and finish with an area-of-effect explosion. As soon as he starts to charge his combo, run outside the range of his attacks and throw two or three (Black) Firebombs, or Lightning Urns, at the boss to chip down his HP.