It seems it all begins the same way... I, RDM42, partied with a BLM yesterday who was, first, doing my job as enfeebler and, second, casting Bio II after I cast Dia II... I told him to not cast Bio as it nullify the effect of Dia but he didn't listen saying they do stack. Tired of this incessant debate between these spells, I hunted the truth... and I found it.
TESTED in Jugner Forest with a Goblin Leecher, GobL, and a Goblin Tinkerer, GobT (took long before I could get their "cooperation"...)
Test 1 :
GobL casts Dia on Janius.
Janius has Dia on him. Att normal ; Def down. Dia icon.
GobT casts Bio on Janius.
Janius has Bio on him. Att down ; Def normal. Bio icon.
Conclusion : Bio nullifies the effect of Dia!
Test 2 :
GobT casts Bio on Janius.
Janius has Bio on him. Att down ; Def normal. Bio icon.
GobL casts Dia on Janius.
Janius has Dia on him. Att down ; Def normal. Bio icon.
Conclusion : Bio > Dia
Final conclusion :
1. Casting Dia after Bio will have no defense-down effect on the target.
2. Casting Bio after Dia will "erase" the defense-down effect of Dia and add an attack-down effect to the target.
3. Dia < Bio
Speculations :
1. The "damage over time" effect will stay on the target, demonstrated by seeing Dia wearing off when Bio is casted.
2. Dia < Bio < Dia II < Bio II (read : Bio II overrides Dia II)
Advice : Cast Dia at the begining of a fight and Bio when the target is at ~1/4 hp.
And do not ever tell me again that these spells stack! And BLM... do YOUR job!
Truly yours,
Janius
RDM Male Elvaan