The ninja guys are working for the nobles that are against the king and are trying to undermine Balud's territory to force him under their thumb. The ones that Balud is fighting have failed to collect any useful information about what Balud is doing in his territory but, they have figured out a couple of things that may help future missions. The other one that had a "hostage" has some information on how Balud is capturing and killing the spies so easily. Balud on the other hand is eliminating every spy, and using the knowledge of his 2 other souls to "create" tactics that are unknown to the world, and he wants to keep it that way. He can't be sure about how successful the spies have been so he absolutely can't let any get away or live, or he risks having the enemy country that is literally right next door learn about them. Currently Balud only has a few hundred soldiers and some mercenaries like that group from the early chapters (around 6 or 7 is when they first pop up I think), so losing even a few could be a massive blow to their overall battle power. The ninja guys mentioned that there are only about 30 of them left, and based on the number that died it looks like they're down to around 9 or 10, and it looks like Balud and his two friends are the only ones that are currently keeping up with them. From what is shown the ninja guys can use some kind of stealth magic to become invisible and Balud countered that with heat detection magic, when they used smoke bombs and suicide attacks Balud countered by commanding his troops with the simplest counters to those available to reduce casualties, and he had explosive traps planted to reduce their numbers and slow them down. Due to those things the ninja's are at a severe disadvantage and appear to be trying a final, desperate, attack to allow at least 1 person to escape while the others kill or slow Balud and his friends.