Just how tatsumaki can bypass an immortal?
The only real way to kill Cyn is to assimilate her by devouring her black hole, because as shown in the series, she can survive without her core without any problem. So how do you expect Tatsumaki to one-shot her? Even if Tatsumaki managed to destroy Cyn, she would regenerate. And you're jumping to conclusions about Tatsumaki one-shotting her without considering that Cyn could very well attack first. Tatsumaki has no defense against vector manipulation, time manipulation, or singularity.
The thing is, Tatsumaki has nothing except her telekinesis, while Cyn has the same thing, but better, plus a whole host of other powers, immortality and regen