I've said this a lot already, but Zelda: Skyward Sword's motion control was really wasted on the boring enemies in the game. Most of them attack maybe once every ten seconds, and far too many feature auto-blocking. That is, it doesn't matter how they're holding their weapon or shield initially, they'll always block at least your first slash unless you can catch them off guard, and you have to force them into blocking one direction so that you can then hit them from another. And Lizalfos — about the only strong enemy you encounter a lot of — are even worse. No matter how you swing at them, they always dodge until you've swung enough... At which point they sit motionless, hiding under their shield with one spot clearly exposed, as if to scream at you, "Swing in this direction to hit me!" After the initial thrill of the motion control wears off and you see through the enemies' gimicks, the result is that combat in SS tends to feel more like a series of button prompts or routine patterns than fluid and dynamic tests of your reflexes.
But then, at the very end, the game throws Demise at you and gives you a glimpse of what could've been. Unfortunately, he still does the auto-block BS, but he also attacks quite often, which does a lot to break the feel of monotonous button prompts. You actually have to pay attention to more than just the direction he's holding his sword, or he'll beat you down while you mindlessly try to get past his blocking. The first round of the fight is close to what I expected Darknuts to be like in SS, except then they went and left Darknuts out of the game for some stupid reason, and despite its flaws, it's genuinely fun and actually somewhat difficult.
The second round is slower-paced than the first, but it's more complicated to make up for that. Even the tired stun-and-slash mechanic that's overused for 3D Zelda bosses is done in a much more interesting way. Because even though you're not directly attacking Demise while trying to get lightning to strike your sword to stun him with, you're still having to balance offense with defense. Offensively, you're attempting to get a lightning charge, while defensively, you're trying to avoid Demise's attacks. And what makes this complicated is the fact that you don't know when exactly lightning will strike, and when it does, you're vulnerable for a short period of time. So if you're not careful about when you raise your sword to try for a charge, you can end up getting attacked right as lightning strikes, which not only hurts you, but also makes you lose your charge.
So yeah, I have to at least give Nintendo props for this fight, even if most of the rest of the game's combat is subpar. I had a lot of fun replaying it and learning its intricacies, which resulted in the below video.
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Played in real time on Dolphin 3.0. The depth of field effect is turned off and other graphics may not look quite right because I had to turn the settings down to get it to run at a decent frame rate.
During the first round, the most important thing to remember is to avoid swinging too quickly. Demise's attacks all have a very long windup animation and they're always either one or two slashes, which makes them easy to block with shield bashes, but you won't be able to shield bash if you're still hacking away with your sword. Space your swings out, and if you see him pull back for an attack, stop and get ready to shield bash.
Demise's blocking can be easily exploited by swinging in the right directions with the right timing. When you first run up to him, swing down, then up, then down again and the third slash will almost always hit. (Up-down-up also works, but I find starting on down easier.) Then just keep alternating between up and down, and every other slash will hit him if you time it right. But again, don't swing too quickly, and watch out for Demise's attacks. After blocking an attack, start the pattern over.
When Demise falls to his knees after getting hit enough, you'll have time for about five quick slashes. After that, he'll block your next slash and then counter with a two-swing attack, so count your hits and be ready to stop swinging and shield bash when he gets up.
After 25 hits, the second round will begin. Don't try to attack Demise until you've stunned him during this phase. While his sword is electrocuted, you'll be hurt if he blocks, and if you're attacking him when lightning strikes his sword, you'll get thrown back and damaged. Instead, stay about two sword lengths away and wait for him to fire a sword beam and lose the electric charge. Sidestep the beam, then raise your sword straight up in the air, like when charging a Skyward Strike, and wait for lightning to strike it.
With your sword charged with electricity, fire a sword beam at Demise to stun him, then move in to attack. Once he recovers, backflip away so you're out of his sword's range, then raise your sword again to repeat the pattern. The trick is to stay far enough away from Demise so that he can't hit you with his sword, but close enough that he won't use his charging attack.
If he does crouch down and then charge at you, lower your sword and get ready to shield bash to block his swing when he reaches you. If lightning strikes your sword just before Demise reaches you, you won't be able to shield bash to block his attack.
After 20 more hits, you'll knock Demise onto his back and be prompted to use a fatal blow. Don't do it. It may be possible to get him the first time you knock him down, but it must require extremely accurate timing if it is because I've never managed it. Each time you knock him down, he'll stay down a little longer, though.
When Demise gets up, lightning won't strike your sword for a bit, and he'll usually get a charge before you can. Again, stay about two sword lengths away and try to get him to fire a sword beam. Getting him to do it seems more difficult than at the start of the fight, however, so you may need to just raise your sword to try to get a charge of your own.
If lightning strikes your sword just as Demise fires a sword beam, DO NOT try to block it with your own sword beam. You're more likely to get hit this way and you'll also waste your charge. Sidestep it as usual, then counter with your own sword beam.
After another 5 hits, you'll knock Demise down again. Count your hits and then stop swinging, because you'll need to be quick in activating the fatal blow to get him on the second knockdown. If you activate it too late, Demise will dodge and then usually get an automatic hit on you while your sword is stuck in the ground.