Gardevoir is well out of range of the time dilation field, it's not a Ren Fuji level slow and is only as good as a decent speed boost, and given how Gardevoir can keep up at least a little with users of Quick Attack and Agility (being a user of a priority move herself) with the good precog and instinctive reaction on her side, it won't be too crazy of a boon for August.
Imprison isn't some sort of ranged attack as much as it is a psychic placement that happens. Speaking of which, almost all of Gardevoir's attacks are telekinetic and won't be reflected by Reflector. Only exceptions are Moonblast, which does get reflected, Magical Leaf, which will just keep on going back to August due to being a homing attack, and Shadow Sneak, which won't work because you can't reflect a shadow and it's purpose it to sneak up on you.
There's also the fact that there's several other things that Gardevoir has in her favor. One, she can boost the heck out of herself while debuffing August. Two, due to Encore, the moment August does something that wouldn't result in a victory (so pretty much if he uses anything that's not Reflector or Copy), he's forced to only use that, so Gardevoir can just go in on that. She has actual duplication on top of Double Team to confuse him. And of course, she could always just mindscrew with Hypnosis and then kill him by draining his life force.
If I had to choose a specific way she wins (without going lolHypnosis), Gardevoir wins via her precognition as well as Encore. Due to her precog, she knows everything he'll do before he does it and can plan around it. If he leads with copy magic, she locks him into it and beats him down with generic Pokken moves with her AP advantage and her precog and instinctive reaction prevent her from taking a hit. If he leads with reflector, she locks him into it, then stays out of range and buffs herself so she can spam Psychic and Confusion. If he starts with Slowing, she locks him into it and just stays out of range. If he starts with fire magic, she locks him into it and he's just screwed.
If anything he has is passive, Gardevoir steals it with Skill Swap, so now she has it and August has his passive replaced to make all of his attacks fairy, which Gardevoir resists.
Or she can just rip open space telekinetically and one-shot him.