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Duel him like he's a knight, blocking swipes and dodging when he sets his blade alight. Nearly all these enemies can curse you, so kill them as fast as possible, don't hang around - be aggressive. Right at the bottom is a corpse with a Hollow Gem - grab it, then head left. Then head down again and deal with the enemy there, along with another who joins the fight. Head down the stairs and attack the corpse (hint: it ain't a corpse). Same goes for the connecting corridor, though this one drops from above.
Re-adjusted damage and scaling for " Simple", " Crystal", " Fire", "Chaos", " Lightning", " Dark", " Deep", " Blessed" weapon infusions that were changed in Regulation Version 1.Now go through the doorway straight ahead, but do so with your shield raised - there's an enemy waiting to ambush you. Lightning Weapons may not have Weapon Buffs applied.
Blacksmith Andre requires the Giant's Coal in order to Infuse Weapons/ Shields with Lightning Gems. In The Ringed City, inside the Shared Grave in the first grave left after dropping down the collapsed staircase (not the ledge marked by purple cloth). In Irithyll of the Boreal Valley, at the end of the main road behind an altar, near the Church of Yorshka (behind where the Ring of the Sun's First Born is found). Received from the Nestling for the Xanthous Crown (once per NG).
Dropped from the Red-Eyed Lothric Knights wielding greatsword. Dropped from Serpent-men (1%) and Large Serpent-men (2.5%) in Archdragon Peak, ( Overgrown) Greatsword Lothric Knights (2%/4%), and Lothric Thieves (1.5%). Can be used to Infuse Weapons/ Shields, adding Lightning damage and granting Faith Scaling, but reducing Strength and Dexterity Scaling.