the benefit of using Elemental Weaken is that it has a cap of +200%. that means it is possible to do more damage when Weaken is over +100%. the down side is that to get over +100% Weaken you need a minimum of 2 spaces on your items. with a Crystal Weaken of +90%, you have a 90% chance to lower a monsters resistance by 90% and it only takes up 1 space.
the benefit of using Ignore Resist is it only takes up 1 space and monster resistances are not a problem.
you can use Weaken and Effective together if you want, but Ignore Resist can’t be used with either Weaken or Effective, it cancels them out, so you don’t get the benefit of using them all together.