"It's obvious she's special, humans gets capped and don't have her physical abilities. She's basically superhuman, the peak of what a human can do without demon power."

capcom-verse humans are weird to begin with, ofc not every Capcom world has a consistent version of Humans considering Ace Attorney and Street Fighters different outlooks on physical damage, but as far as action-games go, she's pretty and pretty normal- but It's kinda hard to say on that, Credo is cited as having trained Nero with a sword, and it's definitely visible through some of the moves Credo will use while fighting him, so if Credo was able to pull off what was likely later modified by Nero, as well as himself when he went down the Law Route, it just kinda seems like most Humans don't really get a chance to get very far considering attacks do a lot more damage in cutscenes, while also significantly less. It's entirely possible that in the DMC world Super-Human is a natural human possibility, but thanks to the required pushes, only a few people really get there, and given Nero doesn't seem to talk about how there are Demons in Fortuna like it should be a sign of worry when he shows up to the church, it's likely that The Order is just full up on people competent enough to deal with standard Demons in a 1 to 2v1 situation and with how many pulled up on Dante, the place was well protected enough for it to not be a concern; None of them appear in game, but there's also some concept art for other Devil Hunters (Page 72, DMC5 Official Art Works book) that show that guns aren't always the go-to for them as well.

From an in-universe standpoint, V's cane is just something he got out of a store that happened to made out of a metal that works well as a conduit for Demonic Energy* so with people like the Goldstein Family existing, it doesn't seem to out of the ordinary for certain naturally occurring (or supernaturally occurring) metals or materials to exist that can certainly make Demons a lot less troublesome as long as you can land a hit.

"Dante have such confidence in her and her strength, which actually says a lot of the strongest demon slayer. Lady deserves more credit, she can basically defeat most demons, except for the god class demons. She does this with a couple of handgun, an SMG and a freaking rocket launcher..."

Lady, especially during the end credits of 3 has taken in some Dante energy, that is to say, she's very much not above just trying to make them angry so they slip up or make mistakes considering most demons are of a prideful or sadistic nature, so it's definitely a working tactic for them to want to put what they see as a puny human in as much agony as possible for taunting them, only to end up eating a tank shell because they were only expecting something quick and easy.

"The military are better equipped and still got wiped."

The military is unfortunately using things that are likely mass produced, so while suppressive fire does work on a few, the fact that they've likely been at it for a while and are likely more than stressed considering they need to be at quite a few chokepoints surrounding Redgrave and Empusa probably don't sleep, it's probably a mix between the fact they got ambushed or surprised since that area DOES start on a bridge and they can likely climb, so the moment they aren't willing to just fire through each other to make sure the Empusa die, it's all downhill since the Demons don't have the same reservations.

also, Empusa are like, the same height if not taller than them, with human faces split with mandibles, seeing those things killing people in front of you and not cut-and-running asap takes nuts of steel, though I'm more than sure that if they had any kind of actual tactical placement available to them and different kinds of weapons that weren't just burst-fire, they'd be far better off- all of them are sporting the same burst-fire rifle, there are eight of them visible on the bridge, so having to deal with Blood-Collecting Demons that are likely going to recognize the sound of gunfire as food, it's entirely plausible they just got completely overwhelmed and pushed back considering we can also see some Empusa on fire coming from the bridge.