

If you lower the monster's resistance below 100%, you can damage the monster. Monsters immune to a damage type are classified as monsters with 100 or more percentage resistance to the damage type in question. This check was added in version 1.05 before that, Fire Mastery didn't boost Hydra damage. That's why Hydra works with Fire Mastery and Facets. The missiles fired by Hydra have a special-case check of their caster's "+% Fire Skill Damage" ability.

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

How do Traps and Rainbow Facets work together? ( Source.) The version here in this wiki is reformatted and has had numerous links added to additional information and images. This massive FAQ-like compilation of interesting D2 tidbits was put together by Ernir and posted on the old forums.
