DISCLAIMER: I do not own Devil May Cry or Final Fantasy.
The doors of the building flew open, and Dante stepped out. He dropped the corpse of the last demon as he headed down the stairs.
"Whew. What a pain. Why's Lady always dump her dirty work on me?"
Suddenly, a deep voice called out to him, simply saying his name. He turned to see a man with long silver hair, carrying a huge katana, floating above the ground next to the building.
Sephiroth lowered himself to the ground and stepped up to Dante, who smirked at him.
"Hey there. I don't know if you've looked in the mirror lately, but you're a guy. I can't guarantee you always were, but I'm pretty sure you are now. That's a whole lotta goldilocks hair."
Sephiroth stared impassively at Dante, stepping closer.
"You are the son of Sparda, right?"
Dante shrugged. "I am. And you are?"
"I am Sephiroth. It may interest you to know that I already met your brother. He wasn't very smart."
Dante stepped slightly to the side, cracking his knuckles. "That he's not. What did you do to him?"
Sephiroth chuckled. "I left him alive. Mundus seemed quite pleased with that."
Dante glanced to the side for a second, before drawing his gun and firing a bullet at Sephiroth, who deflected it with little trouble using his long sword. Noticing the length of the sword for the first time, Dante chuckled.
"You compensating for something?"
Sephiroth examined his sword briefly, his expression still impassive.
"You should talk. Your own sword is the size of Cloud's, and I understand you have another sword which is much bigger. I think we both know power comes from the wielder, not from large weapons."
Dante nodded, stepping to the side slightly as he prepared his sword.
"Alright. Let's get this crazy party started! I'll deal with that scumbag Mundus next!"
Sephiroth charged, swinging his sword at massive speeds at Dante, who blocked each incoming attack. Sephiroth had observed that Dante was faster than Vergil had been in movement and even reactions, being even faster than Sephiroth himself and having the reactions too. Sephiroth jumped back a step, using the length of his sword to keep the fight at a distance where Dante couldn't hit him. Dante then dashed forward and stepped up to close quarters, punching Sephiroth and sending him into the air. Sephiroth flew up higher, his angel wing growing out of his right shoulder.
Sephiroth flew higher and then swooped down, slamming into Dante faster than Dante could dodge and carrying him into the air, where Sephiroth began slashing at Dante repeatedly, enjoying being the faster of them now. Dante blocked every attack, but was in the air and couldn't fly, and thus was at a disadvantage. Dante suddenly smirked, launching himself backwards and starting to descend, but Sephiroth followed him and caught him, slashing him once before Dante continued blocking the attacks. Sighing, Dante decided to get serious, and transformed into his Devil Trigger, hovering away from Sephiroth and then spinning into him, sending the fallen angel flying back. Dante's greatly increased speed carried him to Sephiroth instantly, and he slammed into Sephiroth, flying up into outer space, while trading blows at a close range that most warriors would be unable to fight at, but that warriors of Dante's and Sephiroth's massive calibre were quite comfortable with.
The two found themselves in outer space, not far from the moon, in about a second, and Sephiroth finally punched Dante back, forcing the red devil to lose its grip on Sephiroth. Sephiroth began swinging his sword, charging using the flight of his angel wing, while Dante did the same with his Devil Trigger's flight. Both were taking bad hits, before finally a particularly solid strike saw Dante dragging Sephiroth through space again. Sephiroth called on his reality warping power, causing a car-sized meteorite to come towards them and hit Dante in the back of his head, and Dante seemed to feel this, but Dante then suddenly attacked at greater speeds than he ever had before, slamming into Sephiroth and hitting him with the full power of the Devil Trigger. An explosion erupted from the centre of their collision, one which would be the curiosity of astronomers for years if not decades to come. When the explosion cleared, Dante hovered where he had been, holding up an exhausted Sephiroth in his hand.
"That wasn't bad at all. A man with guts. Seems a shame to waste it."
Sephiroth stared in a disinterested fashion at Dante, but Dante pointed his hand at Sephiroth, firing an energy shot similar to the charged shots with his guns. Sephiroth remained unresponsive as the shot caused him to fill with cracks, and he continued to stare intensely at Dante.
"I will never be merely a memory. We will meet again."
Sephiroth smiled a cold smile of self-assuredness as he faded into nothing.
Dante quickly flew back to the Earth, landing just in time to revert to human form. The place where he and Sephiroth had started their fight was missing almost all its buildings, and had craters in its streets big enough to be filled with water and called lakes.
"Oh, that's gonna cost me…"
Analysis:
Dante in base form displays light speed reactions in gameplay in DMC3, shown by his ability to dodge and literally block lasers and blasts of light from Beowulf the Lightbeast. In DMC1 gameplay he can outrun Griffon's cloud to ground lightning or more specifically use it like a staircase. Sephiroth in base is slower than that, but with his transformations he is faster. However, Dante in Devil Trigger displayed an ability to outrun light by flying away from it as well as traverse a body of outer space, flying past distant stars in the process. This happens during the Mundus battle. To top it off Dante has numerous time controlling abilities to back his speed up, and is almost certainly immune to time control as Nelo Angelo is.
For power Dante in base was physically stronger than the Saviour, casually blocking a punch from it, and in Devil Trigger he casually crushed Abigail, whose energy projection placed a portal over most of the planet and cleared a thick layer of storm clouds from above the whole of North America, all the while shooting out a series of lightning bolts each comparable in size to a state, and also made several hundred square metres of stone ground in the demon world fall away into a bottomless abyss. While Sephiroth without Materia can bring down a cloud layer it isn't even close to the same size, which really only leaves Sephiroth's meteorites, which are tough to gauge because the cutscenes strongly contradict the gameplay scenes since in gameplay he can destroy the Sun and summon a stream of meteorites in an instant, and yet in the cutscene he needed Materia and preparation time to summon one meteorite and it took several days to arrive.
To clarify, Mundus is shown in the cutscene to call down a stream of meteorites, and considering the fact that Mundus and Dante had flown faster than light themselves and the fact that when the meteorites smack Dante downwards there's no light visible below yet Dante lands next to a pool of lava which hazes the sky around it red, combine these things and chances are the meteorites are faster than light too. Mundus also either created that space dimension or manipulated the reality of the demon world so as to move that body of space to envelop himself and Dante. Chances are it's the former since many of the demons Mundus creates literally have dimensions inside them, and even Agnus, who is regarded as a joke by Dante and even Nero, is stated as having access to a pseudo hell where he keeps all his weapons, indicating he too can create dimensions, though not big ones, but either way it amounts to Mundus being able to warp reality on an interstellar scale.
Dante defeated Mundus, and while it is true that the basic Devil Trigger was completely powerless against Mundus, the Majin form is a big step up, and by DMC2 Dante has access to it. Sephiroth is similar to Mundus in terms of what his powers are, but Mundus operates on a somewhat higher plane, which makes sense when you consider he rules the demon world which is the same size as the human universe if not bigger having once been its other half or more accurately the living universe having been stated as a streak of light surrounded by dark, implying it's actually smaller than the demon world having originally broken away from it.