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1. How do Traps and Rainbow Facets work together?

The -res% and +damage% on Griffon's Eye and Facets do not add to trap damage. With the exceptions of Fire Blast and Shock Web, the Assassin Traps are considered minions by the game. Your minions do not benefit from any -res% or +damage% your character has.

Note that Sorceress Hydras benefit from both Fire Mastery and Facets. This is explained by T-Hawk:

Q u o t e:
The missiles fired by Hydra have a special-case check of their caster's "+% Fire Skill Damage" ability. That's why Hydra works with Fire Mastery and Facets. This check was added in version 1.05; before that, Fire Mastery didn't boost Hydra damage.