Ignore Resist cancels Weaken & Effectiveness, so it can’t be used with them.
my Builds use Weaken +120% if I have at least 2 spaces to spare, and Ignore Resist if I only have 1 space to spare. even though a Crystal +90% Weaken takes up only one space and is actually better than Ignore Resist, if I can’t have Weaken for every attack, I’ll just lower my standards a little and use IR. now that I think about it, I tend to use IR for single Element Builds & Weaken with Prismatic or Ascendant Builds, so I have never really needed Effective Mythic.
I have only recently, maybe in the last few months, reconsidered using Weaken & Effectiveness together. I have done it before, but I usually don’t have room in my Builds for Effective Mythic. I think I’m going to test Weaken +90% & +120% & Effective Mythic and see if they make me happy or not.