this?

I think I can safely say that is a LOT of "Sucks to be you". Is this WBC2 or 3?

The only thing I'm coming up with is some combination of massive luck, instant kill attacks, and save scumming.

This is certainly WBC2.
I agree with the above, though. :(

Man I just had my ass handed to me by that same one too!

My guess is you don't. I had both my TITAN and a dragon take it on together, and they BOTH lost. My hero at the time was level 2. There was LITERALLY no way I could beat her.

I think it's the game's way of modifying difficulty or other depending on your performance.

Wait... That's the AI?! I thought it was another human player using cheats to level the hero to insane numbers.

I hope you have a victory condition besides "kill all enemies" then, because if you can't stop her, then you can't finish the map. o.o

A year later, I ran into her, or at least one very like her (my opponent only had like 87 speed). Massive combat, speed, damage, training (her units started at level 6 or so), could cast spells (she at least had Silver Arrow, not sure what else), 10 second or so conversion speed. Pretty stupid. The solution? Don't engage her until you've got a large stack of high end units, both melee and ranged.

I was using humans, with an archmage hero. I was also near the very end of the campaign (she was among the last 3-4 provinces remaining). My hero stayed the heck out of her way. Tried to keep my units out of her way, too. There were two barbarian sides in the map, and a large force of Elven Hunters separated her base from the rest of us. Of course, the Elven Hunters were allied with her...

Archmages are pretty awesome, because they can get the spells to answer a lot of different things. Archmages can straight up learn Magic Immunity, they can get Resist Fire (Fire Immune), Resist Missile (Missile immune). My archmage cleared a lot of the elven hunters and he could tank against towers to boot, without taking damage. Summon several unicorns for healing purposes, and sprites as both explorers and ranged fliers. Daemon summoning later in the map helps too.

The OP Elf hero ended up wiping one of the barbarians. Arrived at his base, and didn't walk away until he had nothing but two thralls. I start taking attacks from barbarian unit waves on one side, and High Elf unit waves on the other. Two barracks pumped out pikemen as fodder for repelling waves. I had a daemon and vampire from my retinue throwing out bats and imps as fast as their mana regenerated, and two ancient wisps were both building buildings and generating crystal to funnel through my market into gold or metal as needed.

I had to hold out long enough for the Knight Lords and Mages to stack up. 3 libraries were churning out mages (2 working on Red Mages (resist fire), 1 working on White Mages), and then I had as many Stables making Knight Lords as I could afford to keep working (and that I could actually keep not-destroyed, considering incoming waves).

At some point, the High Elf hero got caught up in assaulting the remaining barbarian base, which was well established by that point. They couldn't do any appreciable damage to her, but they were large and sprawling, which kept her busy. As soon as I realized my base wasn't being hit, I took my hero with the knight lords I'd already built (about half a dozen) and the mage squad (4 red, 2 white) and made a push on the High Elf base. A combination of conversion and razing eliminated the high elf base, and I swept around the map until I reached the barbarian base. By that point, I had over a dozen Knight lords, several stacks of mages, and 2 daemons (that I had summoned). She went down surprisingly fast, with very few casualties among my guys. My lack of casualties might have been because she wanted to escape since she had no base and the force was pretty overwhelming; she was stuck though, so she just stood there and died.

It was mentioned few times, but it is good to remember: there is no real protection against assassination.
If encountered such OP hero randomly: tough luck.
But is such hero occurs in Campaign or specific scenario (hard-set) just play with Dark Elves and Assassins will take care of any hero no matter what stats the hero has (yep, high combat can reduce chance to hit, but not in such extend that your assassins would just sway in the air, oh no, they will deliver hits, don't worry)

But if I would face this situation already with race not having assassins, I would rely on a trap system.
Lure hero to engage lows (like goblins, pikemen) quickly cast/build some walls around hero (while he is killing the small ones) and send shooters (preferably catapults)
Unless this hero can destroy walls in one hit... but still, if AI, he would fight the walls long enough to get seriously injured. If he get's out, it's probably over for me. But if he does not, 10-12 catapults, once they start shooting, they are good. Collateral damage would do it's magic.