@Dust: Well, long story short, Sephiroth can resist getting sent to the Void, the space of nothingness that connects together the Final Fantasy multiverse. More precisely, the Remove spell in the game, which is actually "Dezone" in the japanese version, is a recurring ability consistently described as sending opponents to other dimensions and in FFV, ExDeath, the main Villian, uses it to banish one of his subordinates as part of the story. Sephiroth is completely immune to said spell. This is important because nowadays FF is considered part of the same multiverse and many of the same lore, abilities, spells, summons and elementsa really are the same through the franchise, just with variations.For example, there were never Fire1, Fire2 and Fire3 spells that were changed names in FFVIII, they always were Fire, Fira and Firaga "mistranslated".
@Casualty: It's... actually a viable option, Reality Warping is within the power scope of the Triforce which Ganon used in A Link to the Past and as shown in Skyward Sword, the Triforce can be weaponized to eliminate someone. The issue is how in the story it needed heavy nudging for it to be considered for that use, as you say.