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The Sunset Kite Shield has a value of 100 against lightning, thus making it very useful against The Third Lamb.
The Bag of Earth, which you'll have to deliver to Fern for him to give you this shield, can be acquired before fighting The Third Lamb.
You simply have to say "Yes" to both of his questions to be given this shield.
The combination of Lightning Attacks + Poison Tail + Aggressive Attack Combos suggests that this boss's design is meant to evoke the Sanctuary Guardian from the Dark Souls DLC.
Just beat him on level 36. Used "Teuthis Shield" to block the physical attacks, then backed off for a few seconds to recover stamina. Light roll through the lighning attacks.
Is absolutely manageable this way imho!