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I mean, to me that description sounds like my party trying to kill the enemy party that was holding a spot. The enemy was overgeared so we'd kill ourselves several times against them, but after trying different approaches we might be able to get a win, cause either rng was on our side or the enemy slipped up (which in pve's case would mainly just be ability/mechanic rng being more favorable towards players in a particular run).
And unless the game's pvp balance is completely out of whack, there's usually at least a few parties on the server that can beat yours more often than not (this would be the "when they finally get a kill, it is still fairly normal for any subsequent kill attempts to take a dozen or more attempts" part), especially if there's RPS balancing.
And, just as you said, there's always the potential of the other side bringing in more people, so even if your side is at the very peak of power - you can still get into situations where the odds are against you, while the fight also requires you to be at your best performance.
And the "winning" is always subjective. I don't care that in pvp "a player" has won if I was the loser. To me there's no difference between "a player" winning against me a "a mob", because in both cases I am the one who lost. And until I win against whichever target I chose, there won't be "a win" state.
And it's the same for the other side if they lose. They might've gotten zerged by some huge guild or a boss rng triggered several wipe mechanics one after the other and they didn't manage to avoid them. Both instances would probably feel "unfair". And if the boss doesn't have that kind of rng, that'd mean that it's scripted and it'd be way easier than pvp.