Vader also happens to be Force Jesus. He literally is a demigod. He was born with divine blood in his veins, forcing Obi-Wan of all people to relapse to the "it's over 9000" meme. Vader's immense strength in the Force means he could snap Frostmourne in half with his brain. Not to mention the suit grants him superhuman strength and protection, to the point that a nuclear grenade exploding in his face did nothing.

He'd lift Arthas into the sky, then make his armor cave in him like the way the Witch King of Angamar was when Aegwynn stabbed him in the face. Vader can do that with his mind. And Arthas' undead minions and dark magic won't do jack shit since the Force allows Vader to be immune to magic and a Force blast could literally wipe out the undead like it did the Wookiees in The Force Unleashed.

Also, even Mandalorian armor can't stand repeated lightsaber barrages. The Mandalorian Wars were proof of that. Entire armies covered head to toe with Beskar, and a bunch of renegade Jedi bring them to their knees. Frostmourne's power stems not from its composition, but its ability to steal souls and its dark magic. But a lightsaber levels the playing field. Vader's armor is safe from anything but another lightsaber. Arthas' armor isn't safe from solid metal, let alone a blade of energy.

Vader would eviscerate him.

The Force grants a Jedi master immunity to magic, as well as the ability to create telekinetic blasts that clear entire forests. Having played WCIII, KelThuzad and AnubArak's weaknesses are known to me.

You also forget the fact that he's been in that suit for twenty years. It limited his mobility.......during the first few weeks. Then he got used to it, and suddenly he was faster than any Jedi. His first duel in the suit was a haphazard tragedy, with the Jedi he was fighting making circular and graceful movements, while he was making stiff slashes that even a Padawan could dodge. But he still killed her with brute strength. Then he trained with the suit, and proceeded to kill five Jedi the next time he went to battle, despite them being just as fast as the one he killed before. Not only was he faster, but his Force sense was such that he could guess their moves in advance and knock away their sabers with sheer force. Read Dark Lord: Rise of Darth Vader.

Vader also has the power of the Force, which allows him to float, throw his lightsaber, crush things into a pulp with his mind, and even use the Force to increase his speed. Vader came into the suit like a quadriplegic struggling to even walk, but by the time he got used to it, he could literally Jump high and punch through like the Bionic Man. Battlefront II shows this in spades, with the Dark Lord being able to levitate, and use the force to spring across the battlefield like a flying car.

Kel Thuzad is weak up close, and AnubArak's massive size makes him a big target. The Dark Lord would use the Force to power through Thuzad's spells, then rip him in half. Then he'd use the Force to pull Anub'Arak's lower and upper halves apart, or considering that Arak has no magic weapons, just chop off his claws then chop off his head and slice his carcass in half. Then he'd duel Arthas and knock Frostmourne off his hands then decapitate him.

Hell, if you're bringing in flunkies, then say hello to Lumiya and Starkiller. Lumiya is a female Sith who plotted with Vader to kill the emperor, and aside from shooting energy out of her hands, is also fully bionic like Vader, and uses a Sith Energy whip that uses a coil of cortosis with a energy whip complimenting it, allowing her to disarm Arthas before rending him to shreds. She became the new Dark Lord of the Sith, and even brings down the revived Emperor Palpatine by using one of HER apprentices, an ambitious Dark Side super soldier in the royal guard named Carnor Jax.

And Starkiller? Bitch pulled down a mile long Imperial Star Destroyer from the sky. Bitch kills Rancors for a living by frying their eyes with his lightsaber and them frying their hides with ridiculous amounts of Force Lightning. Bitch takes down giant walkers single-handedly, then slices them in two or crushes them into a cube with his brain.

Not to mention a plethora of forces that complement regular Stormtroopers. Aside from hiring Fett now and then, Vader's own ship has shadow Stormtroopers, which, like the night elves, can appear and disappear with hit and run attacks, but unlike the night elves don't need the moon for that and carry guns that low giant flaming holes in people. And there's even the Dark Troopers, which are huge androids designed to kill Jedi. The only way the rebels beat both is with sniper-rifle-toting guerrillas.