IRL humanity does not have the tactical or strategic edge necessary to beat the wizarding world, nor do I recall any WOG statement that support your argument. But yes, Karna should be more than strong enough to defeat any HP wizard or witch that comes across his path. The question is whether he can find them all before they hole themselves in a pocket-dimension or in a property protected by the Fidelius charm.
Karna wins every battle but loses the war, he cannot be fought by anything, but wizards have various means of tracking, hiding, and teleporting around the world that means once they go on the defensive he will never find every single one out of a million+. Nevermind hunting down every magical insect in the world and so on.
I keep hearing this over and over, this insane cope in Spacebattles when it comes to Harry Potter and I STILL do not understand where the hell they get these takes from.
So I decided to look up the Fidelius Charm and what do you fucking know:

Fidelius Charm
The Fidelius Charm (incantation unknown) was a very powerful spell used to hide things or places, or to conceal secrets. It was an extremely difficult, multifaceted, and potent charm that could be used to conceal a secret inside an individual's soul. The witch or wizard who housed the secret was...
The Fidelius Charm (incantation unknown) was a very powerful spell used to hide things or places, or to conceal secrets. It was an extremely difficult, multifaceted, and potent charm that could be used to conceal a secret inside an individual's soul. The witch or wizard who housed the secret was known as the Secret Keeper.[1] A dwelling whose location had been protected by this spell was then invisible, intangible, unplottable, and soundproof.[3] This was an extremely old spell, one of the most ancient of all.[4]
The Fidelius Charm was used to protect James, Lily, and Harry Potter from Lord Voldemort at their home in Godric's Hollow, the location of which was the secret entrusted.[1] They designated their Secret-Keeper as Sirius Black, but then switched to Peter Pettigrew at the last minute before the spell was cast as they thought it would be less obvious.
Lily and James had refused Albus Dumbledore's offer to be secret keeper due to them preferring their friends, which proved a deadly mistake — Pettigrew betrayed them to Lord Voldemort, who found and murdered the Potters, with only Harry Potter surviving the attack.[5] Harry's survival would henceforth dub him the "Boy Who Lived".
The Fidelius Charm was once more employed to cloak the base of the Order of the Phoenix at 12 Grimmauld Place. However, in 1997 when Albus Dumbledore (primary Keeper) was killed, the members of the Order of the Phoenix automatically became Secret-Keepers for the HQ. This included Severus Snape, whose allegiance was doubtful at the time. As such, the Order was forced to change their headquarters to The Burrow.
This charm was used on Muriel's house (Arthur Weasley as Keeper) and Shell Cottage (Bill Weasley as Keeper) during the Second Wizarding War.
So only THREE PEOPLE in canon used the Fidelius Charm and surprise, surprise, 3 of them are dead and the last one is unknown but somehow, every Wizard will be able to master this extremely difficult, complex and multifaceted spell... to save themselves from Karna... in a week.
A reminder that I'm virtually certain it took Harry the better part of a school year to actually use Patronus and Hermoine Granger like 2-3 years to be able to use Apparition without a Port Key but somehow, everyone can learn to use the most ridiculous spell ever... because reasons...
This isn't even getting into the typical NLF trap that SB always triggers because hilariously, extremely potent Bounded Fields exists and strong enough Heroic Spirits can... sense them and destroy them:
Servants vs the sort of Boundary Field Kayneth set up during the 4th Grail War.
“Then, the Einzbern homunculus and Miss Borzak are helpless?”
“Not at all,” Faldeus immediately contradicted his subordinate as if his earlier praise of the workshop had been a lie.
“It would be one thing if they’d gone in alone. With the Heroic Spirit Haruri summoned, however, it’s a different story. The workshop in Fuyuki I mentioned was destroyed along with the entire hotel, but if a Servant well-versed in magecraft had challenged the labyrinth, they would have broken through it sooner or later.”
Faldeus’ opinion on this matter was the opposite of what it had been ten years earlier. A modern labyinthized workshop was difficult to penetrate; even a Heroic Spirit would need to exploit a weak point or back door, he had thought. Now that he had accessed the data from the puppet his ancestor had left behind — the record of the third Fuyuki Holy Grail War — and had contact with the Heroic Spirit Hassan-I Sabbah, however, he was in no doubt. A maze of that level would not work on a powerful Heroic Spirit.
Although I doubt that weak Avenger in the record of the third Fuyuki war could do anything about it, Faldeus thought as he turned his attention to a monitor.
“In any case, if she summons her Servant to break out of the workshop, it will be a golden opportunity for us to observe its abilities.”
Nor is this getting into... what are they talking about with millions of Wizards?! Where the fuck are they getting these numbers from as if there was millions, Grindelward's Rally should have had a fuckton more than the few we have seen or was the case in the 3rd Fantastic Beasts movie.
If there was millions and HP Wizards "can just defeat modern humanity", then why did J.K. Rowling and Fantastic Beasts 2 heavily disagree with that? They was sweating their asses off over World War 2 technology for fucks sake including Grindelward himself.
Not even the Harry Potter movies portrayed it as Wizards was beyond Humanity, let alone the Books. Like I get this is the typical Spacebattles nonsense but the constant bringing up the Fidelius Charm alongside lying again about how "Wizards can just loldominate humanity" finally got me to just see what it is.