Isn't infinite speed like insane? How would they have a chance when a low 2-C is mauling all characters a million times a second?
Dusknoir doesn't necessarily know Bunnyx exists, & she's usually in the future, not the present, not to mention she has her own "Rabbit hole", a "pocket dimension", from which she is able to monitor timelines, & usually a huge threat is why she comes to the present. She's also able to open portals to any place & any time. (Can't remember if it's done via her umbrella or not.) She can also take other characters to said Pocket Reality.
If someone went & killed off Ladybug & Cat Noir, she'd likely realize there's a problem & time travel into the past
But of course, Infinite Speed, meaning if she goes to BEFORE where he arrives, they need a way to beat Dusknoir BEFORE he can take action with his Infinite Speed.
In theory, there MIGHT be someone who can do it, I'm just not sure who; Every Akuma has a unique power, & Scarlet Moth can summon & control multiple Akuma at once, & many are pretty haxy. I'm just not sure who has what's necessary.
Though, in theory, Dusknoir might be able to be tricked into entering an AoE, or stopping briefly long enough that he's affected by one.
Ex: After time travelling to before Dusknoir's arrival, set up a message leading him to where they're hiding inside a locked & shuttered building, with a sign ready, with a confusing &/or small point font message so that when Dusknoir gets to them, he'll see something weird to read (He's stated on his profile as having read tons of lore, so I could see him stopping to read a strange sign.), & voila.
&/or for example, a folded up piece of paper saying to (not) unfold it; If he were to unfold it infinitely fast, wouldn't that just destroy it? Either way, it being folded paper would encourage taking it slowly to prevent destroying what he's trying to read.
As long as he stops long enough to read or think it over that a FINITE amount of time passes (Or at least, he spends enough time for the ability's AoE to reach Dusknoir.), it'll be enough.
Lies doesn't have a Key in her character's profile yet, but she creates a huge AoE around her that paralyzes anyone who's ever told a lie.
Similarly. so long as he's touching the Cataclysm, the Destruction effect will probably might kill him (AFAIK, Dusknoir doesn't have Dimensional Travel.) before he can find out about & get to Bunnyx; & in this case, having an Earth-encompassing cataclysm might actually be safer, since Ladybug's Miraculous Ladybug ability (Just have Bunnyx get another with her time portal.) can undo damage from Miraculous & Akuma's, IIRC. Dusknoir can't destroy the planet/kill everyone for it to not be fixed if everyone else is already dead.
Still, this would probably be extremely risky, & unreliable; I'm just unsure if there are better Akumas.
