Can't dualcast starfall, correct, though it's 4.6x damage, so even single casted it outdamages every dualcasted nuke except a dualcast ultima/meteor (and it's far cheaper mana).
A dualcast ultima does 8.4x damage for 120 mana. Fires from below does 2x damage + 2x stacking per turn for 25 mana, +20% with imperil.
Dualcast ultima will run a mage out of mana on turn 4 (without outside help)
Fires from below is effectively infinite use due to Emperors 10% refresh regens more than 25 per turn
With imperil, Fires from below will start to outdamage ultima on turn 4 (or sooner with a "good" imperil like from Marie or Seven). Starting on turn 5, Fires from below becomes the hardest hitting attack on global. Turns 6 and beyond it becomes ridiculous. Can it cap a chain? I don't know the animation.
Emperor is by far the best mage for long duration trial fights that don't resist fire. If you want a burst damage mage, he's not as good as Fina, though you could just equip him with ultima too...
That said, he's still a mage, though he doesn't have mana issues like every other mage. He also just got enhancements in JP.