We didn't cast it in battle with the Tsun-Demoness. Did we think it was useless against a foe like this? Was the princesses' bat-wing armor a variation of this spell?
You had only time to cast a single spell at that time, so you chose one that'd act as a distraction and a setup for your next attack rather than turtling up and hoping she doesn't hit hard enough to overcome your defenses.
Is there a way to reliably gauge an opponent's magical defense?
Demonic Eyes of Mystic Perception.
Or you could see if there are any analysis spells to learn. There were none back at the Academy, it tends to be more of a trial and error thing. Failing that, you can try to create your own scanner later on to gauge power levels.
How does this spell interact with others? Can it be cast in conjuction with Petrification to, say, lower the requirements of success to Rank D or less? Or with our Mana Breach to bring down greater spells?
What happens if we try to maximize Gigadyne?
Spell power can vary depending on the caster's intelligence, mana levels, mood, posture, chant, timing etc. If, say, you get the words a bit wrong or a bit too slow, or woke up on the wrong side of bed that day, your Petrification could even fail against a Rank E. Spell Maximize guarantees that the spell is cast at maximum efficiency regardless of all of those factors, at the cost of more mana.
It is already incorporated into Gigadyne when you created the spell, so Gigadyne is always maximized.
How draining is it to cast? We are barely standing, and yet we are able to jump away. What is there to prevent this being our - or our nemeses' - 'get out of jail free' card? Though there were mentions of anti-teleportation defenses, but those were preventing teleporting to a place, not out of one, and were not very effective (Lobelia passed them easily, and our Gigadyne overcame the Academy countermeasures).
The drain depends on the distance. The anti-teleportation defense of the demons was spread thin to prevent direct incursion from the kingdoms outside of their sphere of influence, but this means that you can go short distances within the field before you encounter enough resistance to stop you. But that is with Lobelia, who is an archmage and has an A-rank teleportation spell, which you don't. Gigadyne bypassed the Academy countermeasures because it isn't teleportation magic at all.
How does one able to overcome it? The Beast Lord has a sense of smell, but what did the princess have? It is an extremely useful spell, but seeing how the only cast of it was a failure, I am starting to get concerned.
They can cast Detect Invisibility, of course. Or any other higher tier spells of detection which can only be countered by having higher tier invisibility spells. The princess has her demonic eye ability, which naturally allows her to see past your bullshit.