Since I dont know much about Kain and the others, I'll go with what I do know for now..
Pyron's speed and strength will most likely be the largest distraction for Kain.. Same as with the Barbarian, Pyron is one of those characters that doesn't give his foe any rest.. Assuming his soul isn't drainable, Pyron will to a degree keep Kain occupied..
Sephirot is a badass character, which proven high skills with the blade.. His strength and supernatural agility will too have Kain being forced onto defensive.. However, if this is the Advent Children Sephirot, then he's quite vournable and I'd say that concludes him from the threat factor, as Kain possess a vast amount of power..
Cloud's omnislash is horribly dangerous if he is at his prime.. As seen in the game, he had to use only one to finish Sephirot..
Ryu's devil incarnate blade would be quite badass, but unfortunatly I think it's over-rated.. Ryu along with Master Chief will be the lowest characters on the threat factor.. Ryu may be fast, but the other characters has what he lacks.. His devil incarnate may make a difference, but I doubt the black dragon will do much soul attacking on Kain.. The soul, is from what I have learnt "the homefield advantage" for Kain..
Kratos fought the greek Gods.. An impressive feet, and a clear threat factor.. However, a flying Kain will quickly exclude him from the fight.. Unless he has his pegasi..
As I mentioned earlier, Master Chief will probably be the least factor.. He exceeds probably Dante, Ryo, Cloud, Sephirot, Sorceress, Druid and Amazon in strength, but he is as good as powerless.. A direct impact by the fuel rod, and a neat stick with the grenades might do the trick, but once he have ran out of ammo and nades, he's not much use.. His assault rifles are limited, and he's no match to Kain without his gear..
Illidan Stormrage, at his prime.. He may have lost against Arthas the deathknight, but he'll do fine here.. Illidan is a master of all magic.. He is partly "magic omniscient".. He can not use all magical, but he knows them.. Kil'Jaeden made sure of this.. In a way, possibly the greatest threat to Kain.. He'll analyze the weaknesses, and due his intelligence he will probably calculate a way to defeat Kain thanks to the others in the team.. He himself may not do much when it comes to actual powers, but he's a strategical mastermind and an expert of magic and rituals..
And his physical strength could possiblt be meassured with Kain.. His size exceed Kain which might give him an advantage.. And a disadvantage.. Either way, he'll be a big factor in the fight.. If not in actual damage, he'll be a great threat trough mind.. He did capture and keep Magtheridon imprisoned, claiming himself as lord of Outland, unchallanged..
If Kain attacks Illidan and he survives, comes the funny part.. The enraged Illidan will be more dangerous.. The closer Illidan is to death, the tougher he fights.. If Illidan at some point take demon form, not only does he grow bigger, but he'll become ranged and gain strength of unimaginable level..
Dante and Vergil.. I dont know much about them, but if they can put up as much of a fight against Neltharion as he claim to be, then I would far from exclude Dante from this fight.. But since I know little or none about him and Vergil, I wont even go there..
The Diablo heroes.. Clearly underrated at this forum.. Their as good as elemental immunity and high defense makes them very resistant to all attacks that is from the physical realm.. They may lack the Necromancer and Paladin, which would make this battle totaly different, but the Amazon for one can do a whole lot of damage..
For a starter, the Amazon doesn't miss.. Her guided arrow will hit, and it'll penetrate and go back into Kain, penetrate and go back in again.. This is one hit, by what I assume will be BOTD bow.. She'll fire a circus of 3 arrows each second, all homing to Kain, passing allied without harming them.. Her freezing arrow might slow Kain down, and this is only the amazon.. Who also has a circus of 70% to dodge all attacks, physical, elemental or spiritual..
The Barbarian.. I doubt I have to mention that he's; After Pyron, Kratos and Illidan, the one that can take the most beating of these.. His strength is enormous, as he can wield two twohanded swords at once, swinging them like they were wooden sticks.. If he uses Doom and BOTD in dual wield, he will both slow and damage the foe at matter.. He can jump over a 100 meters, and he can go into frenzy.. He will also increase all team-mates health and mana by loads of percent, with his shout..
The druid, with his Grizzly, Spirit of the Oak/Bard, a life-draining creeper and possibly a raven, he'll be enough to make a well distraction.. As for the next step.. He'll be able to constantly trough the entire battle absorb all elemental damage caused to him, and keep both vulcanos, hurricanes, meteors and balls of molten lava active.. Not to mention his beam of frost..
That is only his elements.. He can also turn into a werewolf, which too can cause quite some damage.. Especially with Fury and Rabies..
The sorceress.. Dangerous in the way that she can untireable spam spells that thanks to her skilled arts in combat will only harm the chosen foe and other hostile nearby.. Her different elements may be one factor, but the greatest danger is possibly her static field.. She'll reduce Kain's health in..
Watch this 😊
Percent..
A big step in order to defeat him..
So, my conclusion would be that, that Kain is fighting numbers over his head.. Not exactly weaklings, any of them.. Even if he decided to drain everyones soul, it wouldn't take many seconds before he was worn out from all attacks.. Pyron, Cloud, Sorceress and Illidan causing the most damage, imo..
Of course, I dont know much about everyone, and I'm currently reading the Kain respect threat.. This is just my speculation..
Kain might be fighting the numbers of, but he'll loose soon enough.. He'll simply be DROWNED in attacks..