Kill him how exactly? The profile doesn't elaborate on her offense all that well.
Space cutting, time BFR, subatomic freezing... or vaporize him, I guess. Or she can develop something for use against him using her knowledge + what she might learn in the past. She isn't a genius for nothing. Once spent years developing tech to defeat an opponent.
And that strategy isn't working when a causal sweep of coresense at this stage can cover everything around him close to a light-year (and even peer into pocket dimensions) and activate mind, memory or soul attacks at that range.
QiQi can built devices that can land you in other solar system, so she could easily send bombs and stuff in regardless. Heck, she could just place a bomb where he will be standing in the past and have it blow up in the present.
Additionally, he doesn't necessarily get the time to do that sweep... honestly why would he even do it when not knowing an opponent is there?
Lastly, QiQi is a robot. So, mind, memory and soul stuff is questionable.
Most of Ning's current stuff was learned at Mount Innerheart anyway. And that place exists on a different dimensional plane entirely than standard space-time, precisely to prevent info gathering and spying in this manner.
And I'm not even sure how this would play out. It's not as though she'd even be aware of where to start. If she travels back in time on Earth then Ning would be half the Three Realms away.
She can just follow him and his companions around. The neat thing about being unable to change the past is that she can't be discovered... and she can get into separated spaces. Teleportation devices can get to spatially separated places.
Would this stop his essence from acting independently though? It's abstract and separate from his mind and consciousness. Also, does she have like, Acausality? Alternate clones? Because I'm not sure how she's doing this while she's destroyed in the present.
Timetravel in the verse can't change the past, so no time paradoxes can occure. If she BFR's him to the past the same would presumably apply to him... of course she could also just BFR him into the future. And what do you mean destroyed in the present? She isn't dead yet in the present. Her future selves hit first. And the future versions do all of that on their own. No action from present QiQi is required.
Not sure about essence. It would prevent his brain from working, so can it do stuff while that is the case? Can it do anything notable in particular, given that it prevents any other matter from getting moved?
Hmm? He freezes space via law-hax and even thought-based stuff fails. That Rain Domain is in his earlier key.
Freeze space? So rain stuff and a spatial paralyzing technique?