Archimonde wiped out Dalaran in a matter of minutes just by playing with sand.
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You're wrong by putting such emphasis on LK vs Lei Shen. Lei Shen was more powerful than Titan-keepers and admired the pantheon. He enslaved whole civilizations before Arthas was even conceived. He even manipulated the Zandalari, the first to ever step foot on Azeroth. At his peak he would of easily destroyed the Lich King and Arthas/Ner'zhul/Bolvar story is by far the best in game for me.
As commander of the Burning Legion's armies I would imagine Archimonde is an excellent strategist in terms of lore - it's doubtful he could attain and keep such a lofty position in the Burning Legion hierarchy without the power and intelligence to back it up. Kil'jaeden is more the spymaster and military intelligence type than Archimonde, in charge of recruitment and the greater scope plans while Archimonde manages the situation on the ground during the Legion's many engagements on various worlds. Both are equally necessary roles, and without one another the Legion's armies can't function properly.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
As far as I know, Archimonde was a brilliant military strategist/tactician and highly intelligent, considering his position in the mages guild of the original Draenei and later on the Triumvirate. His downfall was, that, in his cosmic-level power and eons of successfull war campaigns, he just underestimated the danger that seemingly insignificant mortals can pose (Speaking of Warcraft 3, not WoDs One-Shot-Sh*tshow). And if I can remember the WotA-Trilogy correctly, there wasn't one time in the whole book series that had Archimonde struggle significantly with armies, magics or foes. He snapped Malornes neck, conjured winds to blow away an entire battlefield worth of soldiers just to beat Jarod Shadowsong with his fists, he finger-of-death'd a great dragon that flew towards him, he re-invigorated the protective barrier the Highborne had errected around the WoE, all while not seemingly breaking a sweat. Not to forget his Dalaran-shenanigans and the attack on Mt. Hyjal. That's why I think he could handle himself very well against the scourge, since he seems to have been a master field commander, intelligent tactician and a downright ludicrously powerful spellcaster. At least that is my impression with him and KJ - even the mightiest Legion Generals seem to have been rather insignificant when put up against those two dudes. But that's just my impression. *shrug*
Can we stop arguing about Archimonde vs Kil'jaeden and speculate about Archimonde vs Scourge
The entirety of the Scourge vs. Archimonde alone on a single featureless plain? I would say the Scourge would win through sheer attrition, being thousands strong and quite capable of overrunning even a giant-sized Archimonde and/or exhausting his fighting stamina. Most of the Scourge would die in the attempt in all likelihood, and Archimonde may go out in a retributive strike that would ensure mutual destruction. Now if you're referring to a Legion army headed by Archmonde against a Scourge army captained by the Lich King I would say that Archimonde would ultimately win after a hard fight.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
most of this is speculation from the fanbase and not actually in the lore. Fans speculate and claim canon and then other fans take that and believe it. Many people used to believe that KJ was more powerful than Archimonde for so long, it's never been proven and now we're finally seeing people saying the opposite.
Archimonde is the left hand and Kil'jaeden is the right hand of Sargeras. Archimonde is the head of most invasions while Kil'jaeden recruits and crafts plots. From that, I would think that Archimonde is more powerful and Kil'jaeden is more cunning... however, we're learning little information here and there that point that Archimonde might have been more cunning than Kil'jaeden who is more... zealous?
Archimonde betrayed and killed his master to become the 3rd leader of the Eredar and knew of the demons and Sargeras before the 3 met him. He basically is the reason the Eredar joined Sargeras and all of this, without Velen or KJ's knowing.
It's also stated that in Warcraft 3, he wanted the 2nd Well of Eternity to become as powerful as Sargeras, which is a betrayal to Sargeras(in reality, Sargeras was canonically dead and it wasn't a betrayal at the time) and he was never punished for his attempt. Kil'jaeden who later seeks revenge on the Scourge for their hand in defeating Archimonde either was ok with or unaware of what Archimonde planned to do.
In WoD, Archimonde berates Kil'jaeden's obsession with the Draenei and only approves of invading Draenor for the souls to harvest(better priorities than KJ) and is the one who sends Gul'dan to Azeroth(something KJ seems to be not happy with). Archimonde and Sargeras seemed to have plotted invading Azeroth through Medihv, but KJ was all like "Look at these green guys i just enTHRALLed, lets use them instead" which backfires and Sargeras and Medivh die and Gul'dan tries to steal the power for himself. Then KJ tries to make up for that blunder and tells Archimonde "Don't worry, i got another green guy who wont betray us and Ill have him become a Lich and make way for you" Which Archimonde is like "We're totally getting betrayed, but whatever" and he gets summoned but then gets betrayed! Kil'jaeden is a dumbass
So yeah, I personally believe that Archimonde is more cunning than Kil'jaeden, kil'jaeden who was sold on Sargeras' sales pitch whereas Archimonde already was aware of Sargeras and Demons and still accepted because survival and power. Where Kil'jaeden obsesses with Draenei and later Orcs, Archimonde focuses on Azeroth and resources. KJ's plans have also failed, causing the Legion major losses and his pawns keep failing or betraying him(like illidan). I feel like KJ has more book smarts and Archimonde has more street smarts.
A speculation of mine though, that I'm 90% sure is to come true. Archimonde isn't dead and just like he tricked everyone before, he's already aligned himself with a new master. I think he's working with the Void Lords and Old Gods and is going to be the one of their champions in the future Void Expansion.
LeiShen in his prime would defeat the Lich King(who was still getting stronger).
Arthas/Lich King in a few years toppled the ancient Nerubians, Vry'kul and then the strongest human nation, the High Elves, Dalaran and tore through the Night Elves territories, betrayed the Legion and cost them the world(in doing so, saved us all) then beat the combined force of Blood Elves and Naga led by Illidan, while kicking Old God minions ass. Then kicks the shit out of possibly the strongest Troll group out there who have gone the longest untouched, forcing them to kill their loa for power and pillaging the Dragon Wastes lol'ing at the dragons futile attempts to stop them and almost succeeding in resurrecting Galakrond.He enslaved whole civilizations before Arthas was even conceived. He even manipulated the Zandalari, the first to ever step foot on Azeroth.
Lei Shen is cool, but what he did took years, possibly centuries and his empire was weak without him. The Scourge in just a short amount of time became the most badass group to ever exist on Azeroth. They dont need years to topple enemies, they fight everyone at once and almost destroyed all life on Azeroth(still can). Empire vs empire, Kosak said Arthas would tear his empire to shreds.
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Imagine the tree as the Halls of Origination and Archimonde as LeiShen. They need to make an cutscene of that
Yeah, so I've looked through quotes online, conversations on lore ranging from comics, the RTS games and novels and I couldn't find anything so that was just me using discussion board banter probably and not recalling where I read it and thinking it was something official when it was likely just fan interpretation. So obviously I take that back.
It's still obvious though Archimonde has been beaten, IN-GAME, like 3 times, WC, BC and WoD. Not just beaten, the dude was KILLED all 3 times. So I stand by my point, his track record speaks for itself.
He did need a ritual to annihilate Dalaran, which just lost a number of Archmages to the Scourge, and in reality it's more like he damaged its most important buildings. With a ritual and an artifact Illidan would have been able to shatter Northrend, obviously he's a lot less powerful by himself.
As for the OP, Scourge + Arthas easily. Not only is the Lich King powerful, he has access to mighty servants like Sindragosa, Kek'thuzad, Anub'arak and a bevy of Death Knights, master Necromancers and so on, on top of the endless chaff he can send at Archimonde.
It's been mentioned before (in recent lore) the main reason the Lich King wanted to make everyone into undead is a unified front for when the Legion returns. The Lich King has no love for the Legion and are definitely enemies.
So thinking on that line, since the Scourge was an idea to protect Azeroth from the Legion, you would think all of them would be able to take on one, albeit powerful, demon.
Depends on if we are talking about just dropped in an open field and yell go get em at each other then scourge easily. Now if like open planet no time limit sort Archimonde. If he can remove icecrown citadel then the lich kings power would seep away like when Illidan did it then an eredar lord vs mindless undead is pretty straight forward.