*
"Do you understand? There are those who create rules like I do, and then there are those that bend them like Kaori, Arasovi and Long Yi. The ones that violate, infringe upon will usually have an upper hand in battle, but what exactly keeps the balance that keeps us from culling each other?
Killability. As long as rife, hardship and disaster exists, I will continue to be. " (Hikari Alternative)
" Then that makes it easy then? It's not a rock paper scissors situation but a fire and water relationship, then I just have to be the one bending the rules. " (Hikari)
The first thing that comes to mind is targeting Alter's existence with her own [Disaster]. At its core, Hikari's [Disaster] is anti-humanity magic or antimagic that inherently corrodes at the human existence. It wouldn't work on her other self here, but that was only its origin as a whole, it could still induce side effects as per the specifications of [Disaster] allows. The side effect would be the 'undesired state' or 'malfunction' of anything it comes into contact with.
"Hmpf!" (Hikari alternative)
Alter rushed to attack first, Hikari reacts in time and slams her fist into Alter's, resulting in their knuckles colliding violently, tearing apart the space around them. Their bodies were so deeply coated in [Disaster] at this point that Kaori would have trouble recreating her victory in the past. More than an infinite amount of antimagic has been circulating around their body, strengthening their flesh and toxicity to normal magic. This might have been why Ledia's magic chose Hikari as the solution to Alter, as she was the only individual strong enough to reach the realm of gods and fight with antimagic at the same time.
"... Did you think you could win just by overpowering me?" (Hikari)
She points her finger at Alter, she felt like her heart could collapse at any moment, like a frail tower of cards. One could say she was just putting up one last facade. In truth, she wanted to weep, she wanted to ball her eyes out at the unfairness of the world. Was she so wrong to want her own kingdom to survive? Even if she hated the citizens, even if she hated everyone and every species on earth... It's because it was everything she had. There was nothing more to it.
However this was nothing different from the usual. She had always been putting up a facade, so much so that it became real... Yeah, it was real to everyone who never knee her true nature, it was even real to herself. That is why, it's fine. It's completely fine to lie and deceive until her last breath, revel in the pleasure of obscurity and spew poison so corrosive that it melted the universe. Yes, dissolve everything in [Disaster], every last bit of existence.
"...?!" (Hikari alternative)
Even as her Alter, she felt dread. Their [Disaster]'s are of slightly different natures, if one of them were to attain victory by any means, it would have to be related to [Disaster]. Alter's [Disaster] is of divine retribution meant to strike down any and all existence, Hikari's [Disaster] on the other hand was about [undesire], malediction. What Alter sense was an application of her final skill.
"[The Great Disaster(True Awakened)]" (Hikari)
A vibrant red light not unlike the Ruby hero's brilliance consumes the entire universe at once. It was lively, saturated and brought about elation and vigour, it was the opposite of despair as it lifted her own mood up. Maybe, just maybe, she had a chance. A chance to do it all over again. Maybe a path towards peace would have been accepted. It would delay the inevitable conflict between humans and demons for their descendants to deal with it... The way it seemed, the world didn't have enough resources for true peace, it couldn't sustain 2 sentient apex species.
Crime rates would only soar, conflicts would happen as an eventuality. War is unavoidable, a small spark is enough to dissolve a millennium of peace building. It's simply something that could never be solved. Then... She's a coward for even thinking about negotiating a truce. She would only be running from what her ancestors started... However, it's not wrong to say it had worked once in the past. When the ancient era of myth was no longer, and humanity was the dominant species for the past few thousand years before The Great Disaster... There was no interspecies war.
Her other ancestors went into hiding instead, and it brought relative peace. Maybe it was possible after all. There was so much possibility. Her magic filled the world they stood in. For a single moment, she felt invincible.
"Shit! Shit! Shit!!!" (Hikari Alternative)
Alter's vast and endless supply of magic was slipping from her grasp, like a container of water with a hole in it, leaking terribly. She could feel the phenomena she created, the distortion of the world invoked by her will, her magic, destabilising as she spoke. Akin to grasping and scooping at water with one's bare hands, she flopped and struggled with a vigour of a beached whale. The laws of the world weren't really under Hikari's control. Hikari does not control, rather she makes things go out of control, like an actively applying debuff that makes things go awry for the enemy.
Alter's control over her own magic was still there, it was just going haywire and spasming beyond her will. Nothing was forcefully taken from Alter, rather this ill intended state of her magic was given instead. A misfortune granted to demotivate or irritate the opponent. It could never kill Alter, but it did serve to make her angry. However, it was also never intended to kill Alter, after all it was just a debuff. It should be paired with a strong attack that makes use of the debuff.
A strengthened anti magic that would consume a weakened one, that would be the appropriate response. Swirling the red light she painted over the world, Hikari converges it at once, causing the light to overlap and darken into crimson. Then she pushed the heel of her palm down and grasped at the air tightly, a dark crimson blade instantly materialises. With one step, she arrived behind Alter in a battle stance. In that one moment she had swung through Alter's body, but there was no sensation, she couldn't be certain if she actually hit Alter or not.
"Slow. Before you try to hit me." (Hikari alternative)
A large gash opens up in Hikari's waist, it was a smooth cut that tore and divided her organs apart. Blood dripped out of her wound and fell into the endless grey void before the wound closed up promptly. They were God magical girls who have ludicrous rates of regeneration and immortality, the very basic principle of dealing damage faster than the opponent can regenerate still applies.
"-You need to make sure you CAN hit me." (Hikari alternative)
This time it was Alter's turn, her figure relocated behind Hikari, having moved at speeds beyond everyone's perception. Except... Her movements did not evade the notice of either Hikaris. Moving her arm up to intercept, then swinging to the right, Hikari turned around to face Alter as a large wound appeared in Alter's body.
" I can say the same, epitomy of biological perfection, [Demon King constitution].
We're both physically gifted and wield anti magic. Unless it's your final skill, I'll be able to replicate anything you do." (Hikari)
"Tch! Don't get cocky!" (Hikari alternative)
Summoning a scythe of dark magic, Alter swung at Hikari, engaging her into a series of exchanges. With immediate swift movements, Hikari slammed her sword into the scythe's blade and steps closer into Alter's personal space. The weakness of using large scythes is that even though they have a large range, the blade does not cover the area nearest to the wielder. As long as Hikari steps closer, the scythe will be rendered ineffective. On the heel end of Alter's scythe, 2 blades split out forged of darkness. A slick black mass sliced through the air with ferocity. Spinning the scythe, Alter catches Hikari in the centre of the scythe's handle and cuts her up into ribbons.
"...?!" (Hikari alternative)
Both of their first skills gave them 'perfect biology' and turned them into the ultimate living being, this included equipping them with the ability to use darkness magic while Alter's scythe was made from 'darkness', Hikari's blade was made from [Disaster]. After repeated collisions, the scythe was made to transition into an undesirable state... The blade had bent out of shape, and some of its edges turned blunt. Hikari walked through the ineffective attacks, resisting with the toughness of her body alone, and bisected Alter in half.
Alter staggers a few steps, healing from the bisection. No good... The laceration caused by Hikari's blade had left [Disaster] on her brain physically, causing a new permanent state of disorientation. But thanks to her perfect biology, the very state of disorientation was almost nonexistent in Alter. If Alter didn't have such a trait, the attack could have caused her mind to develop numerous mental disorders that would impact her combat ability. Compared to that worst case scenario, a little disorientation was nothing.
She stomped into the air, force radiating out and swallowing everything in the surroundings. Her aura swelled and distorted the [Disaster] that Hikari layed over the world's surface. forcefully, physically, Alter regained her composure by brute force. With a swing, Hikari was punched far into the distance. Alter chases after her, surpassing Hikari's falling speed and rains blow after blow onto her body. A right hook and then a left, she spins her body and slams her leg downwards onto Hikari's body, flies downwards to watch up and kicks Hikari into the sky.
Swinging another punch, it misses and Hikari rushes closer and slams her left knuckle into Alter's face. Raining two punches, her blade rematerialises in height palm after losing it, then she swings a few times. Alter retaliates with her fist, but the blade deals more damage after violently thrusting at her fists. Puncturing Alter's skin and bones, Hikari swings horizontally, sending Alter her flying, then Hikari speeds up after her and thrusts it into her torso.
Her blade impales Alter, goading Alter into using her own [Disaster] once Hikari was trapped. Divine retribution at civilisation, her magic in the shape of lightning erodes at Hikari who is the last representative of a civilisation. Hikari swings her blade at the cherry red lightning, causing it to go haywire and split off into random directions that no longer contacted her body. She then inches closer and swings her left fist at Alter.
Alter catches it with her palm, coats her fist with [Disaster] akin to a gauntlet, an smashes it into Hikari's face. Hikari's skull caves in with blood spurting everywhere, but she forms a drill of [Disaster] that split off into countless tendrils before launching at Alter. The long extended drills swerve through the air as they chase Alter around. Hikari catches up to Alter before the drills do and swings her blade at her. Alter vanishes and reappears further away having dodged the attack. Hikari vanishes and reappears nearer to Alter, swinging her blade once more, which Alter parries with her gauntlet before the drills caught up, forcing her to push Hikari off and retreat.
Pushing her foot off the ground, Hikari leapts and lands behind Alter, swinging her blade horizontally once more. Alter's figure fades into the air as her real body surfaced behind Hikari and launched a counterattack. Her gauntlet crushes her ribs and tears through her heart and lungs, before cherry red lightning arised from the point of impact and ravaged Hikari's body, burning her insides thoroughly until not even ash is left. Hikari bears it with sheer will power and swings her blade one more time, tearing Alter in half horizontally.
Alter's [Disaster] stopped its attack as her magic was concentrated on healing her wounds, the 2 recovered back to full strength instantly, and without batting an eye they resumed the fight. Alter launched her left fist in a jab that tore through the world, and Hikari slammed her sword downwards into it with all of her might. The collision caused ripples of white in the grey realm they were in, forcing them into a stalemate. Alter's [Disaster] was divine by nature, it was effective in eroding anything in existence, yet that very effectiveness was undermined by Hikari's [Disaster], allowing Hikari to survive every attack that landed.
In the end, Hikari's magic was channeled into both debuff and attack magic. While Alter's was purely attack magic. Eventually Alter's gauntlet caused a crack in Hikari's sword, which then enlarged and fractured into countless shards. Alter's fist past by the ironically malfunctioned weapon and strikes into Hikari's body.