Ignore Resist vs. Weaken & Effective Guide by Mid (as of Patch 2.5)

No it does not effect the resistance (it changes the element to the usual weakness), but the usual weakness does! Therefore, Effective does not increase the damage by 25% - that is definitely wrong!

If it would work like that, my observations would have been different.

This is exactly what happens. The interaction with Weaken is consistent with -25% monster resistance. Otherwise the damage values would be different with Weaken+Effective together.

600*1.25 would be 750, not 904.

Edit: although I still don’t know what happens if the “weak” resistance gets resistance because it is an epic monster.

Oh, and while you’re at reporting bugs :relaxed::

Equality is bugged somehow, and Skilled Mythic does not show in the damage screen.