u/Gomoyel

So first, let's say the most obvious thing: there a lot of "broken" characters in Konosuba but mostly they balanced by being pretty stupid or having weird obsession.

So Vanir: can at least for a couple of weeks see future and past for the most characters; almost immortal due to bonus lifes and actually still being in hell; physically strong on a level with dragon; as Mage on a level with Wiz; have unknown amount of curses among which a one that can prevent target from using magic (at least on Megumin worked); can summon or create a whole army in a short amount of time; Vanir-style death ray and probably a lot more.

If he wanted to be a proper demon king general, and not to create a dungeon where he will just sit for who knows how many decades, I pretty sure main party won't be able to fully deal with him even with literal godness on their side. Also he probably would pretty easy turn Kazuma into the demon king side

Is there a realistic tactic to fully counter him?

Ignoring everything positive here.

So Aqua is behind the reincarnation of guy who destroyed a country and created crimson demons, the most harmful cult, main reason why Wolbach is DK general. And it's only the first things that came to mind, also there a couple of times were hints that Axis cult and crimson demons is on of the reasons why war started, even the DK himself may be just some guy reincarnated by Aqua.