I read that in area level 75+ monsters have 75 res penetration. that means when I do empowered monoliths timilines I need to have 150% resistance? so with 75% penetration from monsters I still remain woth 75% damage reduction?
Penetration doesn’t care about your overcapped resistances. If you are at 100% resistance, penetration only cares about the bottom 75%, and in a level 75 area, you will be taking a net 0% more/less damage.
Because of how it functions, typically you don’t need more than 60-65% in any resistance, so long as you have other forms of layered defenses.
Having higher % resistances does help against resistance Shred, which will take into account your over-capped resistances, but overcapping does nothing for penetration specifically.
And so far as Shred, goes, typically it’s just better to find some way to cleanse ailments, rather than build extra resistances. Ways to cleanse ailments are abundant, every class has some way to do it, and it’s a mod for belts to cleanse ailments on potion use.
The in-game help answers your question (and much more) too. Also, I think @LividLegends answered this question somewhere else in more detail (check his posts).