The Big Bad Monsterman starts a War Arc after having spawned multiple super monstermen [including a clone of the girl who was the lead guy's rival at the start] so the anti-monster troops were stretched all over and had to use supermoves, including said girl managing to win because the lead guy told her that he wasn't going to rescue her. The lead guy's childhood friend got some backstory revealed where she's been really sore over having been chosen when the lead guy was not and has been mad at him for failing her, but she really does want him to succeed. Turns out the entire point of the War Arc was so that the Big Bad Monsterman could eat her, but the lead guy arrives in time and they kill him.
That spawns the original Big Bad Monsterman [who has the power to destroy anything it looks at] who kills the lead guy for long enough that we can see WHY he was turned into a monster: turns out that the original Big Bad Monsterman originally wiped out an entire human army and their regrets congealed into a single minimonster, which is the thing that went into the lead guy in chapter 1. They give him full power but he'll never become human again, so he takes it and gets back up.
There's a big stupid fight against the Big Bad Monsterman that takes way too long but thankfully not nearly as bad as the fights from Naruto or MHA, but it's still not great. The lead guy deals the final blow and is about to die for good, but the commander [who was eaten by the Big Bad Monsterman before this point and thus could do nothing] restored his heart and humanity before finally passing on.
In the final chapter, everything's back to normal and they're mostly dealing with minor monsters with no risk of a serious supermonster again, and the lead guy is living a good life and still gets to turn into a monster when he wants to.