In those societies where assassination of military generals isn't against local custom (and it often is, since nobles set local custom), it's a very efficient way to solve a war. You kill one person, instead of tens of thousands; it doesn't depend a whole lot on the character of the one person whether that's a good trade (though for various other reasons you generally want to do it only to people who are unusually terrible, or a little terrible and unusually good at it). And this mess is clearly the product of the decisions of one specific person much more than most wars are. Both his supporters and detractors seem to agree that Predain is unified because of Ma'ar, that it began its expansionist phase because of Ma'ar, that the neighboring fiefdoms fell so quickly because of Ma'ar, that the loyalties of its soldiers are substantially to the person of Ma'ar. Urtho's efforts to communicate Tantara's growing hostility were entirely through private communication with Ma'ar; she suspects, based on what she has seen of the man and read of the letters he has, that they were somewhat incompetent, as efforts at diplomatic communication go, but they were also the letters of a mentor to his student, begging him to change course, and seem to have moved Ma'ar not at all on either level.
"Ma'ar is a strategic genius and a geopolitical complete idiot" is among her hypotheses, but far more likely is "Ma'ar knows what he's doing". He soothed Tantara for long enough to build his forces, then provoked them into war by conquering all his other neighbors and conveying only the least credible of reassurances. Tantara invaded, his people unified against their (racist, domineering) enemy, he killed the King and seized the capital, and now he's winning. Urtho said Ma'ar had impressively good intelligence in Tantara; Ma'ar probably has even better intelligence than Urtho knows, because that's not an area in which one shows one's full hand while winning comfortably. So he knows that she is here, she should assume, that the call for a ceasefire is from her. If he's smart and ruthless and possessed with only his own idiosyncratic scruples, he will probably try to kill her, and it probably won't work.
And she should probably try to kill him, and it probably will work.
It's not a perfect solution. There will probably continue to be a war, afterwards, with trust between Predain and Tantara even lower; she'll win that war, she's sure of it, but it'd be much much better to avoid fighting it. And the people of Predain, whose self-conception does seem substantially caught up in being the people of Kiyamvir Ma'ar, will be bitter in a fashion that will persist long into the peace. Killing people rarely discredits them.
(She could descend on him from the sky with a spectacular light show and angel's wings, kill him with a fiercely glowing holy sword, and then give a speech about precisely which elements of his behavior Heaven does and does not approve of. She does not hesitate to speak for Heaven, not when she knows full well what they'd say if they bothered being more interventionist.
But they're not a god-trusting people, the people of Predain, and she fears they'll interpret it as the rich and indifferent powers that always neglected them coming once again to kick the ladder away...)
The alternative is to beat him on the battlefield. That...might work or might not. She doesn't know how to effectively make use of the local kind of mage. She'll have to learn very fast, but she has a lot of flexibility this world hasn't yet witnessed and she can plausibly win most fights singlehandedly. Maybe they'll figure out fast how to counter her; maybe they'll figure out fast how to actually successfully assassinate her, but she's not easy to counter and she's not easy to assassinate. (She wishes she were more sure whether a Final Strike would do it.) Ma'ar is, probably, not an idiot; he won't make concessions while he's winning, but he might well make concessions once he's losing. Or he might get more desperate; he hasn't started using prisoners and civilians in captured territory for blood power yet, but he could. He knows that Tantara's grievance is with him particularly, he can't expect to survive defeat; he can do a great deal of damage in the course of slowly losing.
Yeah, she doesn't like that plan. 'Push the extremely smart extremely ruthless mage with a spectacular number of options for atrocities when he's desperate, to the point of desperation, by beating him on the battlefield' does not sound like a plan that ends with a negotiated peace. It sounds like a plan that ends with her assassinating Ma'ar in a month when the costs are far higher, or being killed herself. It could go well. Iomedae does not, for the most part, make plans that could go well but whose default trajectory is to go spectacularly badly.
She is - has been from the second she arrived here - looking for ways to get that negotiated peace. Maybe they'll agree to the ceasefire; maybe they'll agree to extend it in three days. Maybe they'll agree to a prisoner exchange. Maybe she'll be able to build enough trust fast enough to meet the man face to face.
But the default plan - as it has been since about twenty minutes after she arrived here - is to assassinate Kiyamvir Ma'ar, or die trying. (And presumably get raised back on her usual battlefield. She does not at all believe herself to be outside Creation, not when there are completely normal looking humans going around slinging Fireballs and Lightning Bolts. Her death would be a major loss to Tantara, to this world, but not to her.)
She thinks she can do it, if Urtho has some way to stop the man from simply fleeing. Tar Baphon himself couldn't beat her in melee, and Kiyamvir Ma'ar does not have Time Stop.