With a war going on, and the Hero and Maou on the same side, this was inevitable.
Any time Hero fights, it is a one-sided Curb-Stomp Battle, even if he's opposing thousands of demons.
Deserving special mention is the fight with the Archduke Fire Dragon, one of the strongest demons alive. Hero was short of breath...because holding back enough to not kill the Archduke was so hard.
Maou does a verbal equivalent of Hero's fights on Young Merchant; defusing a potentially deadly situation, convincing him of her point of view, and apparently stealing his heart in the process.
The return of Aurora Island and Gateway City to their respective sides by the protagonists.... on the same day.
Aurora island is controlled by demonic forces under the South Arctic General. The Southern Kingdoms are unable to reclaim the island due to the fact that South Arctic General's army is largely composed of sea demons, which leads to a disastrous attempt at invasion by the humans. The second invasion under the Winter King succeeds thanks to the Demon Queen providing large amounts of salt for his forces to connect the floating icebergs to the island into a bridge. With the demons advantage over sea taken away, the humans can now fight them on equal ground.
The first Aurora Island invasion failed due to the incompetence of White Night King. To make up for this, the Winter King makes Lady Knight the commander of the second. She's able to kill South Arctic General after single-handedly breaking the demons' line of battle.
At Gateway City, Hero (masquerading as Dark Knight) wants the human Crusaders occupying the city to leave without hurting the human or demon civilians. His solution? He and the Fairies use illusions to drive them insane until their commander abandons the city with his forces, ostensibly to reinforce the Aurora Island invasion. They then encounter the retreating forces of South Arctic General.
Another goes to East Fortress General, The Leader of the city's mercenaries, who refuses to follow the Crusader commander and stays to the protect the humans of the city by surrendering peacefully to the Dark Knight.
Big Sis Maid (disguised as Maou) is about to be taken as a heretic. She refuses to be cowed, stops Hero from helping her, and gives a defiant speech about being "human". The church official orders her stoned, and the villagers stone him instead. He attempts to execute her but Lone Winter King and Lady Knight stop him. Lady Knight declares Crimson Scholar as a saint, and Lone Winter King places her under his protection. Finally, the officials are driven away by a shower of stones from a crowd that's cheering on the Crimson Scholar. Easily one of the most powerful and moving moments in the series.
After Maou enters the tomb of her ancestors, she comes out as exactly the kind of being that Hero expected to meet the first time. However, before she gets more than a few steps out of the tomb, Head Maid throws her back in.She lost her left arm in the process!
When the Hero comes in and she's still possessed, they go at it until she reasserts control over her body.
Warrior Youngster effortlessly turns back a surprise assault from White Night Nation with minimal casualties. Then, he pulls a Big Damn Heroes moment and rescues Big Sister Maid from the Commander by allowing himself to be stabbed to gain the opening he needs to strike down the Commander. The hidden steel arrow gauntlets were pretty cool too.
Young Merchant tricked Central into economically destroying itself by letting him buy up all of their wheat.
Witch Girl effortlessly teleporting an entire demon army away from the battlefield to prevent them from invading the human world.
When the mercenaries Central hired decide to stage a preemptive attack, Lady Knight moves off to intercept them. She doesn't bother to put on her armor because it would take too long and she doesn't even need it anyways.
Possessed!Maou versus Hero. Upon blasting his way into the Tomb of Demon Lords, Hero is immediately attacked by Possessed!Maou. What follows is what Hero expected the first time he showed up. Possessed!Maou even pulls the "half of the world" Hero and Maou lampshaded at their first meeting. Hero refuses, and every single one of the past Demon Lords attack him together, and for the first time, Hero goes all out. He obliterates the shadows, grabs Possessed!Maou, and burns the evil out of her. Big Damn Heroes, indeed.