I'm pretty sure Weapons of light doesn't affect Gjallahorn because it has the "grenades and horseshoes" perk hidden, meaning proximity explosion, and weapons doesn't effect explosive damage, only direct impact. other than that, good idea.
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We beat this in 30 seconds with a weapons bubble, (w three g-horns all at level 34) Without the bubble we could barely get him inder half health. Dont doubt the weapons bubble.