@Delu I think the same way. Most of the time, the MC just crushes the head of the snake and leave the rest to be cleaned up by a country's army or something. Or, they're able to use some huge magic to blow up the terrorist organization's HQ, which was housing the vast majority of the terrorists at that point in time because plot. Sure, having the MC go full Rambo is entertaining for a bit, but it gets boring when they're fighting off mook number 17,294.
I much prefer the novels/manga where the MC has his/her own army, or is part of such an army. You can have enemies who are problematic because of numbers and an infectious ideology, rather than individual strength or a single powerful artifact. A mobile death machine is just that,
A mobile death machine. Such an MC wouldn't be able to deal with a simultaneous invasion of their country from three fronts without help, unless the mobile portion means "REALLY fast".
There's a reason there's a Chinese expression that's along the lines of "two hands can't fight four fists". It's because sheer numbers can overwhelm almost any defenses. Oh, your bunker can survive a nuke? Can it survive five? Twenty? Two hundred? A million?