I have always used Ignore Resist instead of Weaken, and dont know much about it, but Golem will most likely chime in and give the rundown on it.
He’s a living, breathing encyclopedia and speaks very favorably of Weaken.
Ignore Resist - you ignore your enemies resistances. on a poison map, don’t worry about the -50% damage if you are using a Poison weapon. also don’t worry about Greatly Resist Element or Immune to Element. unfortunately, it cancels out an Elements Effectiveness (Poison vs. Shock for example) and Effective Mythic. it also doesn’t work in Battle Arena. it only takes up 1 space on your gears.
it can work with any build except for Ascendant & Arcanist combo.
Weaken - it is a % chance to do more damage that is added after Element calculations.
lets use @alphamega101 Crystal +90% Weaken.
on a Poison Map using a Poison Weapon - (1 (All Damage) x 0.5 (Poison Map Resistance)) + 0.9 =
(1 x 0.5) + 0.9 = 0.5 + 0.9 = 1.4. 10% of your attacks are doing half damage and 90% of your attacks are doing 140% damage (x1.4).
if you went up against a Rare monster that is Immune to Poison, 10% of your Poison attacks would do 0 (Zero) damage and 90% of your attacks would do 90% damage.
this is with a Crystal +90% Weaken. this takes up 1 space! Weaken has a cap of 200%. with 1 Crystal +90% Weaken and 1 Epic +30% Weaken, all attacks will get +120% damage after Elemental calculations. but it takes up 2 spaces. 2 Crystal +90% Weaken and 1 Epic Weaken will put you at the 200% cap, but take up 3 spaces. that comes out to +63.3% damage per space taken by Weaken, which is pretty good.
as you can see, when it comes to damage, Weaken is way better than Ignore Resist, but since Ignore Resist only takes up 1 space, you could have other affixes that could do about as much damage as Weaken or more, if the build is good enough.
a good combo with Weaken +90% is having Effective Mythic. this cancels any -50% Map Elemental Resists and only takes up 2 spaces, but Effective Mythic can get hammered by a monster with the correct Greatly Resist affix or Immune to Element affix. you don’t normally come across these monsters very often, except on Pack Size Maps.
@DJC stop! you’re making me !
to be honest, I use both Weaken & Ignore Resist on my builds (not on the same build) depending on the build. on some builds, I don’t want to worry about enemy Resists and Immunities, and on others I want the extra damage from Weaken. and I have to take into consideration how much space my build has for which way I want to go…