Thank you for replying again.
So, now I finally understand that, against any kind of monster (even specials):
- the combo effective + weaken\ignore resist is good.
Effective always provides a 25% dmg boost, whatever is the resistance.
W \ IR increase dmg on their own.
Against players, instead:
- both effective and ignore resist don’t work (=wasted affixes on gear)
Weaken should work normally.