@mommunism
1st of all the Mercs knew what the Grinder was, it's strengths and what formation to use against it, so how come the knights didn't?
2nd, Yes it was the MCs fault that it came down, he acknowledged it last chapter. Also the Captain was the best swordsman in the military, of course since according to the soldiers there wasn't any actual wars going on, his claim is only in their military / court.
3rd, The Mercs already saw that the Knights were using a formation that was bound to fail, and they also split up. When the 1st Knight party was wiped, MC due rushed to try and kill the Grinder quickly, but it flew away.
4th, They had archers, but the Grinder was fast and basically annihilated most of the Captain's group before they even got into formation.
5th, Since the MC had a bad experience with the military demanding that he teaches them sword magic, he opted to not use it, which was a Bad call but kind of made sense, was probably hoping he could take it down with out.
All in all, the fault lies with the MC due, the Lord's and the Knight's stupid decision making. The MC duo is the main cause, but if the Lord relied on the Merc Party to subjugate it, no casualties would have surfaced. Of course that would show how weak their military is at the moment, and lose more face inline with the Lady beating them up and refusing their offer.
Is this cliche? Yes, very much so. Is this a "bad" manga? Not really. The art and gore on Pg.6 was really well done, the panels are easy to follow and clear to read, it's translated well (Thank you for your work!). Is this an amazing manga? No, it's quite average story and character design wise.
Side note; I wonder where the cliche that Knights are useless came from and the infatuation of Asian novels preferring swords while Pole weapons (spear, halberd, etc) were much more useful in melee. There is a reason swords were considered side arms.