I'll explain I'm detail just why I think Twilight Princess' final battle easily surpasses Skyward Sword's.
Ok so to begin with you have two similar concepts in both games: a sub villain and a main villain. They basically play out the same story wise. If you look at it from a rather brief perspective, you have two sub villains who are working for their master and then they move over for the main villain to step in during the climax of the game. So you essentially have the sub villain being the common threat throughout the majority of the game while the real villain awaits.
The whole point of this sub/main villainy combination is to, one, create a plot twist of sorts and to build up the main villain by directly using the sub villain to the dirty work. In Twilight Princess Zant is clearly--well during the first half of the game--the main villain. He is slowly introduced by Midna (and Princess Zelda), which creates a slow build. The build itself was great and they made Zant putty be this seemingly unstoppable and quite creepy villain (a common theme in the game itself). At this moment is was all about Zant until you enter the Mirror Chamber where the Ancient Sages tell Link about Ganondorf and that all of this evil currently within Hyrule was in fact their fault due them sealing Ganondorf within the Twilight Realm. So at this point you connect the dots and realise that Ganondorf is most likely behind Zant. People have this impression that Ganondorf was just introduced out of the blue at the end of the game, well this is quite wrong. Zant gave hints of a greater power being behind him: [ilquote=Zant]"How dare you?! Are you implying that my power is...our old magic? Now THAT is a joke! This power is granted to me by my god![/ilquote] - this quote proves that quite early on (After the Lakebed Temple) that there was another figure behind Zant. We also get conformation of this later on (the same cut scene I mentioned earlier with the Sages). Then you have Midna refer to Zant's power as a "false one", which kind of homes in on the fact that Zant gained false power. These are rather clear hints and their purpose is to give indication of what happened without explicitly stating it. Imagine if Zant just said that Ganondorf gave him power, that would take away the shock factor when it was revealed. The purpose of giving hints over time is to give you more and more information without giving you the full picture. It teases you and draws you into the game - you want to find out what exactly is happening.
Skyward Sword Did a rather similar thing. Ghirahim was the main threat from the start albeit he didn't quite have the same dark threatening presence that Zant had early on in the game. Instead they went down the route of craziness (reminiscent of the latter stages of Zant). Ghirahim's intentions were to capture Zelda (familiar?), the reasons behind this were unclear. The problem I had with Ghirahim was this real intentions were revealed rather late on. I predicted that there was another evil behind him due to kicking his *** several times and the fact that he just didn't seem like a worthy threat. Anyway that isn't the point, the point is about building the anticipation of the main villain which this game did not. Instead of giving multiple hints, it was more so a case of looking back in hindsight and connecting the dots; for example you'd be like "oh that's what that was" and "oh that's why that happened etc. This was the case with The Imprisoned; you had this odd beast which ultimately turned out to be the Master Ghirahim spoke of. This was a nice twist, but you only knew after It was revealed by Impa late on in the game which is coincidently the last we properly see of The Imprisoned. That's my problems, a lot if people say Demise was always present, but really it was only in hindsight and he was present in the form of a mindless shell of a beast that was quite frankly odd. Then you have the fact that we beat The Imprisoned three times in battle, which meant we already had an advantage on Demise himself. Knowing the fact that we had already met with this ultimate evil and conquered it took away the feeling of anticipation when coming to fight him in the final battle.
With Zant you had this really creepy and quite frankly scarily powerful villain during the first 3/4 of the game. Having him freak out at the end signalled the transition of power from him to Ganondorf. All this anticipation I had of confronting and fighting Zant went away as soon as he freaked out, but it made me realise who the true villain was. Zant seemed incredibly powerful until that moment: effortlessly invading Hyrule Castle, clouding he entirety of Hyrule in clouds of Twilight, bringing aside Link, Midna and the Light Beasts with ease, reviving old beasts into devastating bosses and so on. It gave him a sense of hype, I was really anticipating his boss battle while at the same time I was quite reluctant due to just how powerful he seemed. When Zant did flip out, and it was revealed that he was not in but a pawn in Ganondorf's, it just made me realise how strong Ganondorf must be. Plus with Ganondorf you don't need no build; he's infamous in this series and needs no introduction, with a new villain you need to build them.
Ghirahim had a good build of character, while Demise had none as he was just introduced at the end of the game. You could argue his presence in the form of The Imprisoned, but this really had no impact on the Demise himself. The only realistic build Demise had was the during the prologue of the game. What Skyward Sword Did was try to leech of Ganondorf. Everyone knew this was a prequel and Nintendo had stated numerous times that the game hinted of Ganondorf's origins. So with this you kind of expected Demise to connect to Ganondorf even before the whole "curse". Then you have the fact that he is a lot similar: colour scheme, build, look, presence, speech. Of course they are different, but you'd have to be naive not to see the clear similarities. All of this created a a rather brief connection between the two and that's my problem. This game didn't build Demise as they wanted to play off the fact that be is the origin of Ganondorf, which is later made crystal clear with a "curse" of sorts. The curse itself is just totally redundant as it reiterates the balance of power that binds the three figures together (The Hero, The Princess and The Villain). Such balances of power are evident in multiple mythologies: "where there is good there will always be evil". So it almost felt like a half hearted effort to introduce Demise as the origin of Ganondorf and why there is always evil present. They give Demise all these titles as of he is a devil like figures, the embodiment of hated itself and the source of all evil yet they don't meet expectations when building him. If you're going to introduce the source of Ganondorf, and more importantly why evil exists, then build that up and give me some anticipation going into the fight. I had already beat Demise's other form quite simply on three occasions and I had beat his servant also on multiple occasions; I just didn't feel like I was going into the fight as the underdog. Being the underdog is pivotal to any boss battle and especially the final boss, not getting this feeling kind of disappointed me a lot.
So now that I've discussed the build and introduction, what about the battle itself? Well first off I see both battles as rather easy. The Ganondorf fight is split up to four phases with each phase having their own unique tactics, but once you know what you're doing its fine. The Demise fight is really one phase split up into two intensities. It's a simple sword fight, so the challenge of finding out how to overcome Demise of rather simple; you use your sword obviously. The Demise fight was more intense and fast paced, while the Ganondorf battle was a long winding battle going through multiple phases. Demise dealt more damage with one hit, while the Ganon fight went on a lot longer so it was more of an endurance test to conserve your hearts through all four phases as there were no breaks in between. So while both were fairly easy they are quite different in challenge. The ultimate point about challenge that differentiates the two is time in my opinion. Demise can be beat fairly quickly and I did this on my first go which felt quite anticlimactic. Some people may have had different experiences with the battle itself, but I was done in a couple of minutes. The Ganondorf fighting my first time was a lot more challenging in the sense that I had to endure a longer battle transitioning through four different phases. When you have multiple phases you basically have more diversity; it's not the same thing. The Demise battle was just a simple sword fight, a good one, but still a sword fight. The fact that it basically only had one phase really made it feel one dimensional, you didn't have to switch up tactics, adapt to new settings, use different weapons etc. In the Ganondorf fight you first face off with the possessed Princess Zelda, which is more of a fast paced battle which incorporates the famous back and forth game of tennis. The second fight it's more about quick reactions and anticipation of where Ganon is going to appear next. While the next phase you have to use your horseback skills to chase down Ganondorf while also avoiding his attacks. Then of course it ends with the standard sword battle. If I were comparing the sword battled alone Demise wins hands down, but due to the sheer variety, Ganondorf's battle wins easily.
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I just feel as if the Ganondorf battle felt more like an epic final climax. It was an event that decided the date of Hyrule. You started off in Hyrule Castle, after an epic clash of the titans (Midna and Ganondorf), you see Hyrule Castle completely obliterated with Ganondorf seemingly destroying Midna. After an intervention by the Four Light Spirits you then fight Ganondorf across the vast expansive landscape of Hyrule Field which ultimately ends in a standard sword battle. The Demise battle took place in an alternate dimension of sorts and stayed in that one place. See the difference? The Ganondorf fight felt more like an epic climax that was more befitting of a final battle.
Anyway that's just my thoughts.