Okay the whole point of wars had to be fought with armies (meaning powers in numbers) is because in the absence of weapons that allow one person to kill many, nobody can fight off multiple opponents sequentially without rest before reaching the commander.
But all that is out the window if we have someone who CAN effortless fight off multiple people. Even if she can't singlehandedly wipe out an entire army (let's attribute it to fatigue), she sure can kill the closest people she come across her on her way to the general and kill her way out, and we're talking near one shots so there's little to no time loss per encounter whereas this plan wouldn't work even even if you could fight off multiple assailant but it takes 30minutes per bout which as soon as one person can escape to spread the words, the mission is over because they will have time to respond and evacuate while you're still fighting off the first group you come across.
This ought to shift their entire war strategy to smuggle her into enemy territory and have her assassinate from top of the chain. Which would be the go-to strategy except for the aforementioned reason and thus why an army is needed and with an army comes with all the logistic issues such as feeding and housing them as well as not able to hide them in plain sight. As soon as one kingdom obtain an "one person army" all other kings ought to be moved down to the bunker until that person is eliminated else all it takes is one person hopping over a wall and you're dead.