1, 2 and 3 do stack.
Edit: in your case everything will be up but for Open Ring that will proc if Viper isn’t (regarding elemental resistances as I suppose it’s a lightning build)
From above elemental storm doesn’t stack with open hand of mercy,both are sources of flat RR,Viper and Ultos again not stacking.If all happened at same time elemental storm and Ultos are with priority.Arcane bomb and Raging tempest are source of -x% resistances from all of these sources are stackable.Edit got second here:p
Thank you for your posts guys very helpful!
See thats whats bothering me. Viper and ultos do not stack i understand for the priority.
Sigh still confusing tbh. I recall there a very good excel somewhere show which do and di not stack
Im thinking droping ultos in favor of spear
thank you boys very much…
I think I have interesting build, but it is not uploaded to youtube or in base of build compendium yet, now I have pc problems and when I repare it, I’ll upload.
when you see this build, you understand why it is made by stash.
then your assess will be interesting…
Open hand of mercy isn’t flat reduction btw iirc. It’s a percentage value, so it does not stack with Ultos / Viper anyway.
Not sure if this will help (it helped me), but it’s based on how the resist reduction is worded:
if it DOES NOT have the word “Reduced” in its description, it stacks with others of the same type, and it CAN result in a negative number.
Ex. “-30% Chaos Resistance” and “-25% Chaos Resistance” on different gear pieces would stack resulting in “-55% Chaos Resist”
If it DOES have the word “Reduced” in its description AND a percentage%, then it doesn’t stack, and highest one is taken, and this CANNOT result in a negative number.
Ex. “20% Reduced targets elemental resistances for 5 seconds”
if it DOES have the word “Reduced” in it’s description but NOT a percentage, then it doesn’t stack, highest one is taken, and this CAN result in a negative number.
Ex. "21 Reduced targets resistances for 5 seconds).
The total of #1 is applied to the resist first. then the highest from #2 is applied to whats left (if it’s positive). then the highest of #3 is applied whether it’s positive or not.
EDIT: oh and on the subject of Open hand of Mercy, checked mine and it reads, “10% reduced Targets Resistances for 2 seconds” so it falls into category 2 up there, so Djnat is correct.
That sums it up nicely temjiu.
A condensed version based upon all this would be:
Gobble up as much rr debuffs of type 1 (-%, no “reduced”) as you humanly can and get one (preferably the highest ofc) source of the other two. All while not starving your build of other valuable stats.
I always keep in mind:
Can we pin this under guides/tutorials?
Best R.R. guide.
This is the simplest explanation
Simple and the best explanation to know 
Thank you. Goes, btw, for everything.
Also OA/DA shred and “enemy damage reduction”.
There´s no source of the latter with “-%”, so effectively you can only chose one single source (Battle cry OR Empyrion OR Kymon´s Augment OR Dark One Gloves, etc.)
There are 3 types of Resistance Reduction(rr) : -%rr, rr & %rr.
Sources of “-%rr” stack while the highest “rr” and “%rr” gets counted instead.
“-%rr” stacks additively with “rr” and they both stack multiplicatively with “%rr”- @maya The Carnival - A Guide to Pets
Umm…not quite correct
I believe it goes in this exact order according to the latest testing:
Edit: in fact this is the very thread that clarified this in the OP!
“% reduced reductions” come AFTER flat reductions.
That´s why % reduced reductions become more and more obsolete, the higher your flat reductions are (%rr cannot go negative).
For clarity: “-30% physical resist” is actually a flat reduction, of course.
sigh …it is clarified in this thread’s OP. I’ll elaborate so peeps don’t get mixed up with some comments.
and %RR is multiplicative.
This is why %RR is seen as less impactful as you push close to monster 0 RR due to the multiplication.
So 20% remaining Resist with 20%RR would result in 16% remaining Resist. -20% remaining resist would mean -24% resist. Not that significant.
At high levels and without significant -%RR say at 100% monster resist, you’d get the full 20 reduction. (if 20%RR)
Previously it was thought that %RR was last, and as such you could stack up -%RR and reduced RR to gain a significant negative multiplier in some cases, such as some builds with these totaling 150% before the multiplier…which might push another -10% into the negatives (~50% x1.2 = -60%, etc. for a 100 resist monster). But now knowing it is actually 25-30% less due to the ordering, this multiplier is limited to about 1/2 the effect in this example.
Read the original post of this thread.
I dont know if this trad is dead or not, but i hope someone will answer me this.
Does “reduce resistance” and “reduce elemental resistance” stack?
EX: Viper + Dawnbreaker’s Sledge
They are different types of resistances so they stack. But your deduction is wrong so let me explain the whole deal so you can decide on your own next time;
There are 3 types of Resistance Reduction(RR)
Now;
‘‘1’’ can stack with it’s own kind and the other two. The ideal is you get as much as -%X resistance with your damage type, from constellations or skills or gears doesn’t matter. Can reduce enemy resistances below zero so it’s total damage gain than any extra flat damage etc.
‘‘2’’ can not stack with own kind either. The higher value overrides other sources. Can not reduce enemy resistance below zero. This one works differently. To be clear I will make an example;
You have Viper and no other RR. Enemy has %50 elemental resistances. After you hit and apply the Viper debuff. Enemy now have %40 elemental resistances. It means this type of RR is less efficient with high -%x RR or againts low resistant enemies.
‘‘3’’ can not stack with own kind. The higher value overrides other sources. Can also reduce enemy resistances below zero. Works well with ‘‘1’’.
Bigger example:
You have total -%50 Cold resistance. 32 flat Elemental RR from Rhowan and Viper. Enemy has %100 Cold Resistance.
First applied is -%X so enemy now has %50 cold resistance, then % reduced(in this case viper) so now enemy has %40 resistance. Last is flat now enemy has %8 Cold resistance.
Answer is yes, they stack because they are from type 2 and 3 as explained in the post above.
However, this example does not fit your original question
A more fitting example of this would be Rhowan crown proc and Manticore proc. Then the answer would be No.
Just to help clarify the difference between a specific element resistance (e.g. cold resistance) and elemental resistance:
A specific resistance only reduces that element (-20% cold resist only reduces cold resistance)
“Elemental Resistance” reduction reduces the resistance of all 3 of the elemental resists (lightning, fire, and cold).
“Reduced Targets Resistances” reduces All resistances by the specified amount. As far as I know, this includes everything under the resistances section of your LCS. Need verification on this though.