Okay, long post, I apologize in advance:
When killed or destroyed, Sephy doesn't properly die as much as his conscience goes back into the
Lifestream, just having his memories navigating it seems to be enough according to the "On the Way to Smile - Chapter Black" short story. So long a fragment of Sephiroth remain on the world, be it simple Jenova cells, people tainted with the Lifestream he has corrupted or just people remembering him (like Cloud), he can summon himself back. It's a type of Reliant Immortality, and just found out, if this
thread it to go by, even Existence Erasure wouldn't be enough for Link to bypass Sephy's Immortality. Link would have to be aware of how Sephy's resurrection works to fully destroy him (so he can make a wish with the specifics to destroy Sephy and erase all traces of him), which he could with Fi's help, I guess. But in the meantime he grasps that, Sephy won't stand still. Though, my big question about it is whether is this combat appliable or not, given Sephy apparently needs prep time to stage a resurrection.
There's also the issue that has been told several times: Sephy can kill Link with most of his moves, among them: he can teleport for a surprise factor (which he does in-character) and telekinesis, which is also thought based, and he also uses in-character to attack his enemies (in FF7 final battle was the first thing he tried against the party). This is assuming Link, in-character, goes for Triforce wishing as his very first move.
In Skyward Sword, the game where Link actually uses the Triforce to destroy someone: it wasn't by Link's initiative that he used the Triforce that way; not only Demise's threat was built up heavily through the story (Link had fought it twice by then IIRC), when
Zelda revealed she was Hylia and how the nature of the Triforce, she openly told Link he could use it to vanquish Demise given gods can't use it and only someone of unbreakable spirit could . The Triforce being the only choice to destroy Demise is something,
Impa confirms right after Link and Zelda's reunion. When you go for the Triforce, it's pretty much with that objective in mind from before,
the narration (possibly Link's thoughts) even says to use the Triforce to make Zelda's wish come true . Another detail, as seen in the cutscene, Triforce wishing isn't an instantaneous thing exactly,
Fi states Link must focus and wish with all his heart and we actually see how Link indeed closes his eyes and concentrates his thoughs for a few moments before the wish starts to take effect. So it isn't an insta win condition, the wishing takes a few instants if SS is to go by, and the plot heavily nudged him to go for it, so it really casts doubt whether it's actually in character for him to use it first or not. ALttP and ALbW had similar issues where there was a huge mess that can only be fixed with the power if the Triforce in the end, so it made sense to go for the wish right away for a specific purpose.
Isn't Reality Warping a means to induce other types of manipulation? This is a genuine question, because most of the talk is about existance erasure itself not proper reality warping before. I always thought RW was more of a means to an end.
Also if you're that if you're going that "composite" holds for Links in all his appearances in all videogames, it would still lead to a stomp but with Link dealing it, making the thread pointless anyways. Not to mention, using composites to fetch ALL appearances of a character instead of an agreed one is a slippery slope, especially given there is no consensus on Link beyond his official series appearances. This could also apply for Sephy himself and be argued that to make it fair he should also be a composite, which would make him a 2-A for being comparable to ExDeath in Dissidia (in NT he is post-FFV ExDeath) and resist Reality Warping by being able to fight Shinryu in Dissidia NT, which is giving an absurd boost almost arbitrarily. And a discussion would erupt tracking every appearance they've had through media since forever.