The entire sequence at the top of Shinra Tower. Laharl appears seeking Etna and challenges you at Level 1200 in another Hopeless Boss Fight. Afterward Rozalin stands up to him in Etna's defense and Laharl realizes the sigil she produces to prove her lineage with Zenon is a magic seal. He then shatters it, releasing Rozalin's Superpowered Evil Side who No Sells his Overlord's Wrath. And that's not Cutscene Power to the Max, she then takes Laharl on and is Level 1900. Evil!Rozalin has no equipment and attacks by smacking Laharl around with her bare hands, while Laharl is wearing some of the best equipment in the game and his weapon is a Laser Blade. Rozalin not only outdamages him, it's a Curb-Stomp Battle for her. It's not only a Wham Episode, it's one of the coolest sequences in the game.
Axel's ridiculously crazy stunt that he pulls through the entirety of the last chapter of Disgaea 2 instantly promotes him from comic relief to Badass as he manages to get the one up on not only Overlord Zenon, but almost every Dimension Lord in the multiverse simultaneously.
The Disgaea fandom has concluded, upon reading the manga version of the Hopeless Boss Fight with Etna, that Manga!Adell is GAR to the point of absurdity. Yeah, he still technically loses (let's see you take a lvl. 1000+ boot to the head and survive), but not before delivering a kick so hard that it drills the Demon Lord's face two feet into the bedrock.
In the same manga, the Disgaea trio's Big Damn Heroes moment. Zenon captures Rozalin and has this huge Rifle Demon blast away Adell's family. All hope seems lost until Etna shows up and deflects the blast away with a single swipe of her spear. It doesn't end there because Laharl jumps out of a crowd of demons in his Skull robe disguise, burns it off in a way that would make Superman proud, and smashes the demon's face with a single punch. Then, to top it all off, Flonne, who was disguised as a kunoichi, quickly changes into her fallen angel outfit and delivers a barrage of love filled arrows.