This concept was developed to quickly overpower any of the bosses in the game with insane damage output. The special clue about this particular build is that it works on ANY soul level. This enabled me to push into NG++ with a Lv 7 Character (together with a bro playing the same build).
Something should be clear: Characters in Dark Soul don't draw their power from soul levels but from equipment. You should only invest into soul levels to "unlock" new equipment (mainly weapons; more about that later). This guide will teach you the cornerstones to deal 900+ damage per blow.
video, which shows the locations in Blighttown.
1. Power Within. Located in Blighttown. This will increse your damage and endurance recovery by 50% for quite some time but will drain life for the time being. Here is a little cheesy trick with that spell: the spell's life drain is affected by the power of the pryomancy flame you use. The higher the level of your flame, the higher the damage you take. This means that you have to carry a very basic pyromancy flame with you for that purpose! In addition you will need to aquire a Ascended Pyromancy Flame +5 for your damaging pyromancy. Luckily you can obtain 2(!) Pyromancy Flames per playthrough. Laurentius and Eingyi both will hand over a flame if you exhaust their coversations and join the Chaos Covenant.
2. Crown of Dusk. Enter the Darkroot Basin through the Darkroot Garden. After you defeated the Hydra and the Golden Golemn there, Dusk appears. Kill her to make a chest appear in which you can find the crown and the antiquated set. The crown boosts your pyromancy, miracles and magic damage by 20%. This also affects Greater Magic Weapon.
3. Bellowing Dragoncrest Ring. For this Ring you have to rescue Griggs from the Lower Undead Burg. Afterwards he appears at the Firelink shrine where you can buy the ring. With patch 1.06 this ring also affects pyromancies and boosts your damage even more.
These 3 effects stack for an insane damage boost especially with a Ascended Pyromancy Flame +5 using Greater Combustion and Great Fireball. Make sure to aquire 3-4 spell slots with your character
Right Hand 1: The strongest weapon you can wield. Depending on your stats and soul level you need to ensure that you can handle most random mobs with 2 blows.
Right Hand 2: Ascended Pyromancy Flame +5
Left Hand 1: I prefer the Grass Crest Shield here for the stamina recovery. More stamina = better shield block, more attacks, more dodge
Left Hand 2: Basic Pyromancy Flame. Use this only to cast Power Within before you enter the fog gate to bosses
Ring 1: Havel's Ring. You'll likely need this to get below 50% equip load for decent run speed and dodge while having at least some clothes on ^^.
Ring 2: Bellowing Dragoncrest Ring.
Spells: Power Within, Greater Fireball, Greater Combustion ... additional spell slots possible for NG+ if you gain additional copies of the named pyromancies you may need because of the higher HP of the bosses.
Head: Crown of Dusk
Rest: Up to you and your build
Bonus Section: Weapon Guide
The recommended upgrade path:
Lightning Spear (11 STR / 10 DEX / 400 Damage on +5) -> Lightning Claymore (16 STR / 10 DEX / 517 Damage on +5) -> Lightning Zweihander (24 STR / 10 DEX / 650 Damage on +5).
I suggest going for the Claymore asap, because Patch 1.6 drastically increased the stamina use of Piercing Weapons while defending with a shield, making the Lightning Spear a less attractive option compared to the Claymore which will one-hit most of the regular enemies up to NG+. As an additional benefit the 16 STR required to wield the Claymore single-handed are enough to carry the Zweihander two-handed which is insane against slow and or fire resistant bosses like the Gaping Dragon, Quelaag etc. A R2 attack with a two-handed Lightning Zweihander +5 feat. Power within deals an insane 1013 damage to the Gaping dragon per swing.