If you think that Kazuma is weak for being an adventurer, think again as it shows he could do pretty well. It just that his teammates always screw things up as it was shown on Volume 2 of the Light Novel. After Dust insults him for having a harem party and being the weakest member in their group (since an adventurer is considered the weakest class or job), Kazuma decides to trade himself with Dust to prove him otherwise.
At first Dust's three members Taylor, Rin and Keith were surprised and in disbelieve that Kazuma's the leader of his own party due to his adventurer job. But after Kazuma shows off his beginner magic skills against the Goblins, they are convinced that he's strong to the point they laugh about the fact that he uses beginner magic in the most unique effective way. They even apologize to Kazuma for looking down on his job and respect him.
This also includes their encounter with the Rookie Killer, TWICE, which looks like a large black saber-tooth tiger. Their first encounter is Kazuma using Hide skill among the party after convincing them not to ambush it. But the second encounter makes it more epic. It was considered a bad timing since Taylor was injured and they're almost near Axel. How did Kazuma save them? Use Create Earth and then Wind Breath so it would blind the Rookie Killer and run for their lives. It worked.
After the two incidents, they laughed on their way back that they couldn't believe that an adventurer like Kazuma could pull off such awesome moves and have such high luck, seeming to have more respect to Kazuma than their team leader, Dust. As for Dust, he ends up begging Kazuma to trade back to his original team.
Kazuma and Chris attempt to steal a divine relic that Alderp is going to use to swap bodies with Iris. Things very quickly go wrong and they get surrounded. Chris opts to retreat and try again later but Kazuma can't help but remember all the fun he had with Iris and that, her being a princess and him just an adventurer, he's probably never going to see her again. Then, Kazuma, for the first time in the series, gets serious. He then proceeds to go through half of the royal guard and defeat Mitsurugi a second time and is only stopped when he runs into Aqua, Megumin, and Darkness and even then he manages to complete his objective and escape.
His second fight with Mitsurugi is definitely this. After learning that Steal won't be as likely to work, he just uses magic to freeze Kyouya's sword to its sheath, lunges at him, uses magic to shoot water at his face, then freezes his face, leaving him to suffocate if his comrades don't help thaw him out.
Kazuma and his friends crash Alderp and Darkness' wedding, pay off Darkness' debts and escape with her. That is all.
Actually, there's a bit more to it. Turns out the whole thing was a gambit by the resident Magnificent Bastard Vanir. Knowing that the Dustiness household was going to fall into trouble soon he gave Kazuma a warning to continue designing items that would sell easily. Once Kazuma learns of the debt, he puts his creative talents into overdrive, coming up with every idea he can think of and sells the intellectual properties to each and every single one of them to who else but Vanir. Because of this, Kazuma has enough money to pay off Alderp preventing the wedding from happening and creating some very lovely dark emotions to feed on. Even if Kazuma failed in his endeavor, Vanir at the very least got the intellectual rights to some very useful items.
She manages to recognize Ragcraft, one of the Demon Generals, who uses his Doppelganger powers to disguise as Kazuma and kills him with one hit... AGAIN! Kazuma is glad that he consider Iris as his little sister since flirting with her will end very bad.
Other than losing in Rock-Paper-Scissors to Eris, who happens to be the Goddess of Fortune, Kazuma amazing luck status always saves him in the end. Case in point, in Volume 10, during his negotiation in Elroad, Prince Revi was so persistent into not giving financial aid to support the Kingdom of Belzerg that even killing the Golden Dragon doesn't seems to convince him. What forces him to give up? Kazuma asks him to let his party play at the country's largest casino with a free pass. Prince Revi agrees. Long story short, Kazuma nearly manages to bankrupt the casino to the point that Revi and his prime minister spill the beans on why they cut off Belzerg's financial aid.
The web novel's final confrontation against the Demon Lord :
Before the assault on the Demon Lord's Castle, Kazuma uses all his money buying a small hill of manatite crystals of the highest quality for Megumin, allowing her to easily chain-cast Explosion over and over on the Demon Lord's Castle, wiping out his army and breaking the barrier. It is mentioned that at that moment, Megumin became the most powerful arch wizard in the world.
How does Kazuma turn things around against the Demon Lord in his underground dungeon? By casting Explosion, causing the entire place to cave in. Sure, he killed himself as well, but a win's a win.
The Final Episode of Season 1.
Aqua manages to break the Destroyer's magic barrier via what is essentially a Beam-O-War, followed by...
The dual "Explosion" spell from Wiz and Megumin, bringing The Destroyer to its many knees.
Episode 2 of Season 2 has Kazuma spitting on the Double Standard when Megumin tries to invoke it, even giving a speech in which he declares that, if Megumin developed a crush on him and tried to attack him out of jealousy in the event that another girl flirted with him, he would fight back withouthesitation.
(sub) "I'M A GUY WHO DOES WHAT HE HAS TO DO! YOU'D BETTER DO WELL TO REMEMBER THAT!!"
(dub) "CAUSE YOU'RE NOT THE BOSS OF ME, I'M THE BOSS OF ME! AND DON'T YOU EVER FORGET THAT!!"
Episode 5 of Season 2 provides a multitude of awesome moments for Darkness of all people.
One of the mask-monsters exploding is enough to knock Aqua on her ass and cause significant damage. Darkness then immediately walks forward and lets one explode on her... and doesn't even get her clothes scuffed. Masochism aside, Darkness isn't the party tank for nothing.
An equally-understated one is when, dealing with many of said monsters, Darkness tries to kill them as they flock to her... and actually hits a good portion of them. Needless to say, her ecstatic reaction is well-deserved.
Even MORE understated is what happens when Vanir possesses Darkness and leaves the dungeon. Vanir comments on her high strength, stamina, durability, and magical resistances and then, utilizing Darkness' own strengths, proceeds to curbstomp every adventurer and was about to kill Aqua if Kazuma hadn't intervened. If Darkness ever learned to properly attack instead of staying clumsy to sate her masochism, she'd be quite the badass!
The Final Episode of Season 2.
Wiz learns that Hans ate an innocent person and proceeds to show why she was feared by their army before she switched sides. Her anger freezes the ground below her, trapping Hans in a crystal prison twice, the second time destroying what's left of his giant body after Megumin devastated it.
Wiz can't freeze Hans entirely, so Kazuma devises a strategy to reduce Hans in size, luring Hans far enough for Megumin to use Explosion. He does this in a matter of seconds and without any regard for his own well-being, fully trusting in Aqua's ability to bring him back.
Aqua delivering the final blow on Hans.
Aqua: The sin of corrupting the hot springs of my beloved faithful has earned you certain doom! Seek God's salvation and repent! Hans: It can't be...! Aqua:GOD REQUIEM!!*cue the pseudo-nuke blow*
The second opening shows the party using a combination of their abilities, helping to gather golden apples from a tree growing on the back of a giant turtle. First, they place an iron door on top of a group of explosive rock monsters and sit atop the door. Darkness strikes the rock monsters, launching them into the sky. Megumin casts Explosion to create a massive updraft of hot air, while Aqua uses her hagoromo as a makeshift balloon and cushion to help slow down the party's descent. Kazuma then uses Steal to take the apples, allowing the party to complete the quest.
A Meta-example, the fact there's going to be a movie!
Adaptational Badass hit Sylvia with full force in this movie. In the light novel, all it took to kill her was one fully charged blast from a weapon that was admittedly specifically designed to counter the Mage Killer. However, in the movie, she not only survives but she brings Verdia and Hans back from the dead and assimilates them. What ensues is her unleashing a Curb-Stomp Battle against not just the main protagonists but also the entirety of the Crimson Demon village as well as former generals Wiz and Vanir.
Just the fact that Kazuma was able to easily come up with a backup plan to defeat Sylvia after Plan A failed. Said plan entails:
Having Wiz gather the mana reserves of the entire Crimson Demon clan and transfer them to Megumin and Yunyun.
Megumin and Yunyun delivering a massiveCombination Attack against Sylvia using their respective signature attacks amped up by all the mana. It's also an awesome moment for Sylvia who manages to generate a barrier powerful enough to repel this attack. At least until Kazuma uses the aforementioned fusion to "bring down the barrier in her heart", allowing Megumin and Yunyun's attack to kill the both of them.
The fact that Aqua was able to buff Kazuma's luck so much that it allowed his body to not be completely blown to smithereens, though judging by Eris' reaction, his body still wasn't a pretty sight.
Megumin's final explosion (or rather what she thinks is her final explosion). After Kazuma upgrades the skill, Megumin unleashes an explosion powerful enough to be seen from space.
As Sylvia is unleashing a Curb-Stomp Battle, Darkness steps up to face her, and is actually holding her own. Everyone is shocked until they see she is wearing Vanir's mask. The moment nicely demonstrated just how formidable a fighter Darkness would be if she could actually hit things, as her body was strong and fast enough to take on a giant monster made up of three of the Devil King's Generals, with all the combined power that implies, and who Wiz and Vanir weren't doing much damage to beforehand.