Get the Pyromancy Flame
Upgrade the Pyromancy Flame to max
Reach Quelana of Izalith in Blighttown or Quelaag’s Domain
Buy Chaos Fireball from Quelana
Join the Chaos Servant covenant
Offer Humanity to the Daughter of Chaos to rank up if needed
Use a high pyromancy-adjust flame for best damage
Increase Attunement for more spell slots
Equip gear that boosts pyromancy damage if desired
Cast Chaos Fireball at close range for maximum lava damage