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)

So put it simply the calculation is somewhat unknown as of the moment, but what we do know is that when attacking/deflecting/crippling the main factor used there is Attack, so most likely vanquish is good at crippling and deflecting. Dante is good for strengthen/heal/fortify/etc. It's still a mystery that when dante heals his attack reduction is ignored, so to say with vanquish when deflecting/crippling.

For me in layman terms, when you deflect and cripple the more attack you have the better and most likely defense have little or to no importance at this calculation. While when healing/strengthen/fortify your defense is the main stat and same goes for attack little or to no importance at all. :)

I'm looking to spend my skill points wisely, and I haven't figured out yet if all these are derived from your skill level (I assume they are: defense for health/fortification and attack for crippling and deflect(?) ). Anyone know?

The Warlock is a combination of the Mage & Cleric classes. It allows you the Mage’s Deflect ability, filling the “deflect” bar will prevent the monster from doing massive damage to the party. It also gives you the Cleric’s Heal ability, which heals a large amount of the parties Health/Strength bar.

The Ranger is a combination of the Warrior & Rogue classes. It allows you the Warrior’s Strengthen ability, each use of the ability increases the maximum party strength, and does a minor heal. It also gives you the Rogue’s Cripple ability, filling the “cripple” bar prevents the monster from healing itself.

Rangers & Warlocks at Level 2 provides +5 attack against next-gen monsters and at Level 3 it provides +10 attack against next-gen monsters.

The "level points" showed on the bar in the Monster Class tab right after each battle. My first of each of these battles, so it was about 1/5 of each respective bar. However, now they're gone.

Are these level points missing or deleted? Or do they only display when you're engaged with a battle?