Hooo boi, this is where things get fun, lemme think on this.
This may be a little biased as I'd like Targon to win as underdogs considering their numbers and sizes compared to the Freljord.
Freljord
These guys have much more people, in addition to that some could bring their armies (Sejuani and Ashe, maybe Trundle) however, it isn't stated if their armies have any specialists like Noxus' and can be wiped easily as they are cookie cutter foes and wholly human. We're going to ignore said armies since it's mostly a battle between these specific characters anyways.
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Udyr, Sejuani, Ashe, Braum and Nunu. Despite their strong abilities these guys are easy kills as they are only human, plus most/ all of their stronger abilities are useless in the face of intense sunlight, literal stars, and pantheon's furosity to simply break any true ice. Ashe's ability to scout ahead on her human sized enemies could give her side the upper hand for sneak attacks. But this is proven useless against Pantheon's natural shielding, Taric's divine protection, Diana's ability to shield herself and Leona's own shield. The only one who's not safe is Zoe, but you'd assume for them to protect the child, right? Braum's unbreakable door will definitely prove useful before and after he's killed.
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Olaf and Tryndamere. Though human, these two are demons in their own rights as they can't be stopped physically and continue fighting as long as they're mad. Tryndamere however, won't die until he runs out of anger, meaning he can be taken out fast just by Zoe's sleep unless it's possible to sleep angrily (is it?). Olaf and Tryndamere are great threats but that's assuming the other side can't throw literal stars at them.
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Lissandra. She is strong as hell against fore mentioned sunlight and stars as her ice, Dark True Ice, never melts. However she is also only a witch, meaning she can be killed just as easily as the others. Though her ability to freeze herself or enemies in unmeltable ice is strong, this is assuming she can get close enough to use it. [Working with Ashe's ability to scout ahead this could be strong but counter-intuitive for both sides.] If she wants, Lissandra could survive this whole ordeal by encasing herself in ice [but this could also prove useless, as though true ice can't melt, it **can ** be broken as shown from Gnar's prolonged life and escape.
Demi-gods
This is new territory for Team Targon. Their previous foes could have been from a warm-up to a tiring battle depending on tactics and such. Let's assume it was in the middle and Team Targon has light injuries, collectively losing like 5-15% hp as a team.
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Trundle. Honestly I don't think he'll make it much further than Olaf and Tryndamere. He could trump our aspects with his sheer size but, this is all he has going for him. Assuming he we're to eat (referencing Bite!) any of the aspects it'd be a bad idea. Bringing back fore-mentioned protections and general size difference most could avoid being chewed and stick themselves in his throat, causing him to choke or stay in his stomach. Pantheon could simply tear through him from the inside, which would probably be easier than attacking form the outside as your target can't really fight back or not be hit. Any of the others might be put out of the battle for a while in order to digest and then later walk out of his blue ass. Zoe would probably be scared from such an event but again, she wouldn't have to suffer from it as she can fly outor ** warp away**. He'll be taken down after he's hit by enough attacks. [In regards to "Sunlight? Really?" He may laugh at Leona's ability but it doesn't mean he can't be killed by it...unless he has sunglasses on maybe.]
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Anivia. The ice-bird herself. Her ability to use ice and summon cold winds and blizzards is great... -ly useless. notice her ability. She uses ice, but not True ice and thus she is an easy kill...mostly. Because it's not in her kit or anything, it can be forgotten that she can fly. Thus making her a mild annoyance as she can attack from the air dodging most things, besides of course Pantheon's spears. Ice-bird or not, Pantheon never misses and if enough spears hit her, it will injure her wing, bringing her down. Her death is the type to be super sad in the movies as Patheon brutally murders her, and then her egg.
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Volibear. He's the most challenging enemy here as his chain lightning can destroy Team Targon if they aren't careful. His size is one thing and his battle-prowess is to be wary of and if he isn't brought down fast enough he has the ability to replenish his health and prolong the battle. He is indeed a tough fight. If this were Dark Souls, he'd be the Phase Two with lightning and shit but don't forget, the others are also op and he'd be taken down just as Trundle was. [TBH it'd be a lot hard if he had is Brother to work with him but he screwed up that relationship centuries ago]
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Ornn. He spits _ hot fire_, can call down a lava goat and trample mountains. It's at this point where the fight is truly debateable but in essence he's just as difficult as his brother and essentially Targon's win assuming they're not drained already.
Targon
4/5 of their team members are battle hardened warriors that can destroy legions of enemies on their own. But their biggest weakness is their size. If they're not careful The giants of the Freljord could literally step on them until they die, but let's assume this doesn't happen.
Considering their numbers, they must work as a team no matter how much they want to slice throats(Pantheon) avoid/fight each other( Leona and Diana) or just Goof off (Zoe).
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Diana. Hyper mobile and has the ability not only to command moonlight (which can somehow be weaponized) but also protect herself. Her biggest weakness would most likely be the time of day. While there are some hours where both the moon and sun are out her fighting ability would falter until night strikes, making her somewhat of a liability until so. But don't forget she IS still a strong fighter and a great asset for fighting the first group (Ones and Twos).
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Leona. Her strength with her shield and sword is nothing to scoff at. Paired with her ability to bring down intense beams of sunlight makes her this team's greatest asset as Giant ball of Fire trumps ice anytime. Her main weakness might be her lack of mobility.
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Pantheon. He's a powerhouse but at the same time a liability as he isn't as great against the Freljord's demi-gods. He and Diana can clear the level ones and twos easily. His strength would grant him the ability to help with the higher ups but at one point spears will mean nothing to Volibear and Ornn's armor and weapons. If all goes to shit for him or the whole team, Pantheon alone can call down Aurelion Sol bu this could kill him aswell, so it's a last resort.
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Taric. Another great asset to the team. Not only can he protect the team with astral gems, invulnerability and invulnerability but he is the only healer on both sides meaning not only can he keep his team up mid-battle but he can also keep them healthy and ready for battle after fights, assuming there is any time in-between the battle. And even if Taric isn't busy defending his allies, he is well able to fight on his own.
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Zoe. Like Pantheon, Zoe is a powerhouse and a liability, but to be more exact, she is a glass cannon. Zoe can throw stars all she wants, she lacks armor and much mobility (besides simply flying away) and she could actually be super useful and save the whole team time and energy by throwing stars at the ones and twos until no one is left. While she is immobile, her ability to throw stars and increase their damage based on how far they're from could be detrimental to dealing with the Freljord's demi-gods. But again, she is a immobile, frail, preteen(mentally) so she is best either hanging back with Taric or Leona.
AURELION SOL He is able to be called down by Pantheon and assuming he doesn't completely hate Zoe, her too. His power alone would end this farce in an instant but that would also be the end to both sides so he is generally a last resort. Though, if worst comes to worst, I think Zoe can breath in space, in which you can assume Taric, Leona and Diana can aswell.
Tell me what y'all think, am I accurate or did i miss some details that may turn the tide? Blah blah blah.
TD;DR Targon Wins because the Freljord team is mostly mortal.