"Checking the Daigo fight, but IIRC Flumen and Terra never launched elemental attacks"
Flumen and Terra have Frigid Grave and Explosion, respectively. Admittedly, there isn't much logic to determine which skill is used when for the swords, so it may be luck (or lack of, lol) that you haven't seen them yet.
"Finally saw Demon Grip from Tenebris, which isn't magic, but is at least Dark?"
Demon's Grip is, in fact, a magic skill.
"Their Melts might cost too much for their 16 SP per turn regen to let them do anything else, especially since they only get 25SP back from Soul Infusion. ... Unless I figure out how to get Divinus to toss them a Soothing Light?"
The melt skills have universal SP cost no matter who uses it because enemies and allies operate by the same skill cost rules. I could increase the max SP of the swords, but I left it the way I did because I didn't want players getting so overwhelmed by the swords that they couldn't focus on the boss. The boss fight is a test of the player's crowd control capabilities, so making the sword's AI too complex would cause players to focus less on crowd control and more on fighting individual swords.
"And Swords can't feel emotions. Huh."
Well, they are inanimate objects, lol. They're also immune to physical injuries like bleed, blind, etc. because they lack the components that those injuries would affect.
