Weakness ailment

What does it do you and if I somehow inflicted weakness ailment on a monster, does it increase the damage I deal to it?
Thanksss..

 

on monsters, reduces monster defence by 25%, allow magic attack to critical but only 50% of the total crit rate & dmg will be used. Duration is 5 seconds
On players, increase the skill cost by 100 mp. Duration is 10 secs or higher.
Correct me if I'm wrong

 

I believe it’s just MDEF right? And I think I read somewhere that the MDEF reduction isn’t applied on bosses. So if that’s the case it’s only use is for magic crit really ^^;

 

You also need advantageous element to get 50% magic crit.

 

no you don't.
Element Slash needs advantageous element for 100% weaken chance, otherwise it's 10%.
the effect of weaken itself and the whole magic critical mechanic is independent from elements.