Also frees Asia from becoming a nuke to wipe out the entire leaders of the three factions by applying Clothing Damage to her. Yes, Clothing Damage as an awesome moment; found only in this series.
In the seventh volume, he gets Mjolnir! And he uses it to bash Loki with the combined power of the recently-resolved father-daughter-problem when they cast lightning on Mjolnir.
In volume nine, the party is losing to the Hero Faction, and Kunou's mom is about to be used to summon Great Red. Then Issei activates Illegal Move Trident for the first time, and nearly kills Cao Cao with the first three forms. And when Cao Cao and the others retreat from Issei and the absurdly powerful Son Wukong, Issei shoots Cao Cao in the eye.
Volume 12: He becomes Ultraman huge when he comes Back from the Dead and proceeds to wipe out the most powerful Annihilation Maker's monster, the same monster that ultimate-class devils couldn't even scratch at all. And the only he needed assistance from Grayfia and the rest of the Lucifer team to lift it up so he can blast it out of the way is so that there wouldn't be any casualties.
Although his own powers had nothing to do with it, as he was borrowing from Great red. More of an awesome moment for Great Red really.
He gets an overall one for actually being able to keep up with Issei throughout the series. To put that in perspective: his training with Issei involves Issei trying to deliberately kill him just to make the both of them stronger.
In vol. 6, she and Akeno get in an argument over who gets a date with Issei. When the enemy tries to interrupt, the girls deliver a twin Off Hand Back Hand for their troubles, taking out all three of the attackers at once.
The first OVA shows us what happens when Issei uses Gift on Rias. Damn...
Being The Medic, she doesn't have a lot of these, but the ones she does have are pretty sweet.
She normally stays pretty calm, regardless of the situation, but when Diodora insults Issei, she slaps him across the face.
She is Rias' Queen, and the daughter of a Fallen Angel. She is also a sadist.
In volume 4, Katerea has his arm wrapped around in tentacles and will blow him up if he doesn't get rid of it. The problem is that the tentacles themselves are invulnerable to damage. So what does he do? He rips out his arm and impales Katerea with his spear.
In volume 5, Azazel teaches Issei what it takes to become a first-rate pervert, subverting the usual Hot Springs Episode cliches.
Later, he goes one-on-one against Fenrir, a wolf that can kill a god. He wins and subdues it.
Odin's fight against a number of devils. He just casually taps his staff and they all die.
Sirzechs shows off how powerful the Power of Destruction is on a small scale instead of how Rias uses hers. He just puts a small ball of his power on Asmodeus mouth and kills him from the inside.
Sona almost winning against Rias in their Rating Game when she takes advantage of the rules stacked against the Gremory group in her favor. She almost could've won too if it weren't for Issei's new abilityBilingual.
Saji gets one for being able to keep a single thread alive and attached to Issei before getting taken out of the game counts one as well. To the point Odin praised him and was awarded the MVP of the game.
The original Son Wukong shows up right as Issei finishes pounding Cao Cao. He proceeds to level the rest of the Hero faction in seconds, and blocks the True Longinus, the strongest weapon in the series, with one finger. What.
He may be a villain at this point, but Cao Cao going One-Man Army against an entire crew of angels, fallen angels, and devils while protecting both Georg and Samael counts as one. Take note, he's just a human being. And he was winning!
Sirzechs in Volume 12. Oh boy, making a godcower in fear of him. And it was freaking Hades too!
His pal, Ajuka, also gets to be awesome in this volume and proving why he's The Rival to Sirzechs. He defeats all of the Old Satan Faction devils Siegfried was accompanying while sitting down in his chair.
Any match versus Sairaorg will almost always end in a Curb-Stomp Battle. This includes the successor of Glasya-Labolas, Kiba-Rossweiss-Xenovia team up (though Kiba did get to cut his arm but got regenerated thanks to a Phoenix tear), and Heracles. It's always awesome.
Vali's Juggernaut OverDrive. BiggestHSQ form he's ever pulled off. Pluto, who could fight Azazel evenly, gets curb-stomped by getting himself divided into nothingness.
Great Red's Dynamic Entry to the Underworld with a revived Issei accompanying Ophis with him. Put this into perspective: there's a war between The Alliance and the Khaos Brigade going on with the latter having one monster who really cannot be defeated because it can instantly regenerate its wounds when suddenly the entire sky cracks up and out comes Great Red with both sides not knowing what it's even there for seeing as they all know is that he's usually chilling at the Dimension Gap.