Quote (Motorbreath @ May 18 2013 06:29pm)
since i cannot find a definite answer, 17 cold mastery is the max -cold resist u can achieve on monsters?
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Yes only if base cold res = 0%.
For any positive cold res monster (as most of monsters in Hell), you see a benefit in having more than -100%ECR.
Quote (Motorbreath @ May 18 2013 06:29pm)
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after 17 cold mastery, does 5/5 cold facets do anything to negate resistances?
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Refeer to the previous. An extra -5%ECR is useful (on top of -100%ECR with a flat level 17 Cold mastery) against any monster with cold res > 0.
Quote (Motorbreath @ May 18 2013 06:29pm)
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so would it be pointless for me to socket deaths fathom and nightwings with 5/5 facets if i have 17 in cold mastery?
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In all situation, +5% cold damage is extremely useful for a cold sorc.
Quote (Motorbreath @ May 18 2013 06:29pm)
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and infinity is worthless for cold right?
Infinity removes cold immunity for 100% to 116% cold res monsters. It doesn't help against 117+%.
Against non-cold immuned, it provides -85%ECR (same as 17 levels into cold mastery).
The fact is that many hell monsters have cold res > 116%.
So you could consider other choices (Reaper's toll comes to mind).
But you could also use infinity to spare 17 skill points and invest into a second element.
Also keep in mind that Infinity is not a bad weapon for a merc (insane CB, insane chance-to-hit thanks to conviction, high damage, immunity to hostile pvm conviction...).