Defense skill always improves the secondary abilities (even crippling and deflection)
It is interesting that it is the attack side of equipment that acts as the modifier for cripple and deflect. However it is still the defense skill which improves the effect of secondary abilities.
that being said each monster has a different threshold where the effect becomes noticeable.
once you are above the minimal threshold generals which modify def will modify the effect of you secondary ability.
if your def is below the minimal threshold the general used does not make a difference.
Once you have raised your defense to the values below it is ill advised to use vanquish or ophelia and Xira is right out.
Halcyon is by far the best general to use with secondary abilities followed by Corventhius, Ambrosia, Serra Silverlight then Dante. (Draconius may have the potential to be 2nd best but the chances of getting 25 Griffin Riders within a reasonable number of chest rolls make him generally less useful.
Ancient Serpents require raising defense above 80 (probably 100 without dante)
Dark Legion and Earth and Ice Elementals require 200 def (180 using dante)
based on monster list placement I expect Fire elemental will require between 200-250 def
Red Plains require above 250 def (230 using dante)
Azriels require above 350 def (330 using dante)
Alpha Mephisto require above 350 def (330 using dante)