and if you think they won't responded to a concentration of force like that with there own army killers, things that will shrug off juggernaut wave then your going to be in for a nasty surprise. we are strong sure, but we are very much the David in the coming fight, if we get cocky we'll get blindsided and die.
You make it sound as if we will parade around.
Honestly, if they bring their own Army Killers to Earth, something went definitely wrong; especially because the Unified Light would instantly seize on the troops drawn away elsewhere by starting a counter-offensive while humanity roflstomps however many demons come.
Keep in mind that it was said the Akashic Pillar's entry into the eternal war would have brought its end within a few dozen years. The UD would have fallen from that, had they not won the gamble at Hope.
In short, Cerys will only be one of hundreds of thousands of humans Thirteenth Hell has to fight, with probably a handful of Magical Girls running around.
And we know that there are others capable of bringing as much destruction as she does.
Unless they expend an entire legion (~1,000,000,000 demons for those unaware), they stand no chance. And before they get the intel to find out
where to concentrate forces to manage killing Cerys, they will have been thrown back already and the UL falls into their flanks.
In short, unless they roll one or several crits on recon and get lucky, we will have the initiative.
Supremacy of Man is in effect here, too... and the demons can not even afford to launch an all-out war on a race that might even be willing to join the UD for all they know.