"A-agh... Apparently I didn't counter it as well as I'd hoped.
Dazzling Gleam!" Gardevoir used the powerful Fairy-type move,
hoping that all of the super-effective hits would wear Yveltal
out.
(Dazzling Gleam has 80 power and 100% accuracy, if you're
wondering; the same applies to Oblivion Wing, which heals the user
by 75% of the damage dealt. Mega Gardevoir is better in both
Special Attack and Special Defense, and Dazzling Gleam is super
effective, so Dazzling Gleam should theoretically be doing quite a
bit more damage to Yveltal than Oblivion Wing is doing to
Gardevoir. You should keep this in mind. ^^)
Gardevoir used Protect once again.
(Yveltal doesn't have to recharge, though, since he isn't being
knocked back by the impact. ... At least, I think that's the
in-universe explanation for not needing to recharge if your Hyper
Beam variant doesn't hit.)
"Eep!" Gardevoir squealed, not being able to dodge the move in
time. He was knocked backwards several feet, and found it difficult
to get back up. He used Dazzling Gleam once more.
(I would rather that he wasn't, if only because it would interrupt
his research. ^^')
"EEP!" Gardevoir repeated more loudly. He managed to get back up
and countered the attacks with a great number of Dazzling Gleams.
(Being corrupted by Yveltal would basically mean becoming evil,
right? Currently, he basically ignores all other life aside from
the Pokémon he wants to talk to for his research. If he was evil,
he would be attacking them instead of focusing on interviewing
them, and he would also go out of his way to attack things rather
than simply ignoring them, thus distracting him. Lastly, Dazzling
Gleam, as I described earlier, is basically shooting bullets of
light. Closing one's eyes does not reduce the damage, as even
though it seems like a sight-based attack from the name, it can't
possibly be one because it works on Zubat.)
Seeing the Dark Pulse from across the battlefield, Gardevoir used
Protect to prevent it from hitting him.
(Since you said "First off," and then only said one thing, I think
you posted that sooner than you intended to. Also, Gardevoir would
still be under Yveltal's control, and I might as well just have you
own him, since your character is controlling him.)
(Also, Mega Gardevoir's Special Defense is even higher than
Yveltal's Special Attack, and assuming we are scaling up their
health or something to make the battle last longer - as Yveltal
would have been OHKOed if we were following their game HP - Yveltal
still would have fainted first, especially with Gardevoir's
resistance to Dark. ^^' Yveltal could win if he used actual
strategy beyond rapid-fire attacks, which Gardevoir also did but
with super-effective and vastly superior moves, but unless he uses
some sort of strategy to turn it around or uses super-effective
moves, even critical hits won't be enough to make him win. It is
literally impossible according to canon, since Yveltal has inferior
stats, a type disadvantage, hasn't used a single kind of real
strategy, and is relying on brute force against an opponent with
superior brute force. I think Gardevoir's fainting should be
delayed until Yveltal has a real way to win.)
(Being immune to Psychic doesn't matter if nobody uses Psychic
attacks on you... I also already said that it is weak to Fairy...
Anyway, should Gardevoir still be fainted?)
(According to game logic, Gardevoir can do up to 90% of Yveltal's
HP in one hit with Dazzling Gleam, so I think Gardevoir should be
alive if Yveltal is fainted, simply because they were just using
brute force against one another. ^^' Also, I suggest making your
own character but making him or her oppose Yveltal until he or she
is corrupted. ^^)