My antagonist can turn invisible. Is it possible to fight/kill him?
Yes. Invisibility is not the same as invulnerability. It’s a significant combat advantage, but like all advantages, it’s something your characters need to plan around.
Off hand, two approaches come to mind. You can either come up with a plan that negates the invisibility, as much as possible, or find ways to deal with your antagonist that completely sidestep direct combat.
Negating invisibility depends, in part on how the power functions. If it’s technological, there may be systemic limitations.
Someone cloaking themselves from the visual wavelength may still be visible in the infrared spectrum, or ultraviolet. Meaning you might be able to find them using thermal goggles, or with blacklights. You may be able to disrupt their cloak using a rapidly changing environment, for example with dance club lighting and strobes. If you’ve watched the Predator films, there’s also the possibility that their adaptive camouflage can’t handle exposure to water. Even failing that, it might not be able to conceal foreign objects striking them. Meaning dust, sand, snow, or of course, blood may cling to their body, partially exposing their location.
If they’re only invisible, they will interact with their environment. This means things like leaving footprints, brushing aside cobwebs or foliage. If they’re moving through smoke, dust clouds, or any other airborne particulate matter, they probably can’t conceal that either, so you’d likely see some hints at their movement if you paid enough attention. That same particulate matter may cling to them, meaning they wouldn’t be fully invisible for long. You may not be able to see them, but if you’re looking for something moving around, you should see some traces. Of course, all of this requires that your characters know what they’re dealing with.
Another fun possibility with technology is that they may still cast a shadow. Their cloak may be able to replicate the image behind them, but it probably can’t emit light at the same intensity of the sun, or even a streetlamp without resulting in some seriously strange lighting behavior.
Another possible approach is that light actually lenses around the character. This is, in theory, the technology behind the cloaking devices in Star Trek. So, they wouldn’t be emitting light, directly, just passing it around them without leaving a shadow. There is one problem with this, your eyes function by being struck by incoming light. If you lens the light around an object, it is invisible, because the light you’re seeing will never actually contact the object and bounce off, but it will also render the user blind (while the field is active.) There are ways around this, but the short version is, their eyes (or goggle lenses) need to be visible, or they can’t see. I’m not saying that a pair of disembodied, glowing, red eyes is better, but it is a functional limitation based on physics, depending on how the technology they’re using works. Somewhat obviously, this isn’t a problem if they’re using some kind of chameleon style equipment.
So, this is all technological, but there are harder to pin down options. Magic is open ended and sets its own rules. It may follow physics, or it may not give a damn. So, let’s look at another easy to manage example, your antagonist isn’t actually invisible, instead, like The Shadow, they have the ability to prevent others from seeing them. In this case, most of the things I just described wouldn’t work. They could pass through fog without betraying their presence because your characters are psychically prevented from realizing they’re there.
This comes with a host of different considerations. For one, your antagonist’s ability to remain invisible is directly tied to their mental state and control. If they’re taunting from the shadows, it may be possible (though difficult) to work their nerves in return. There may be other factors they can’t control. This is also far more strictly dependent on your antagonist having full control over their environment. For example: They can’t mask themselves from someone they don’t know exists or a security camera.
That’s the hard way. The easy way is if you have a vague idea of where they are, simply lock them in, or set the building on fire. Sure, they might be able to escape. But, that’s why you lock the doors first.
Invisibility is a strong advantage, but you can work around it. It’ll just take some advanced planning, and some idea of what their limitations are. So, that’s your characters’ first goal, find those limitations, and then operationalizing a way to use those against them.
I cited Predator earlier. It’s not a great film (though, you’re welcome to disagree with me on that point), but it is about an invisible alien hunting film’s most improbably armed search and rescue team. In your case, I’d also recommend the sequel, Predator 2. Set a decade after the first, it includes characters who are specifically looking for ways to circumvent the Predator’s cloaking system. It’s also got a lot of visual fodder to play with for how a personal cloaking device might look in an urban environment.
If you can look past the uncomfortable Orientalism, 1994’s The Shadow is probably one of the most easily accessible versions of a character who masks their presence psychically. It’s also a better film than it has any right to be, even if the CGI is very dated now. If you’re going the psychic or magical route, this one may be worth looking at. To be fair, this is a character that’s been in print for almost 90 years, so it’s not like there’s a shortage of material to choose from. However, the ’94 incarnation just happens to be a very good, period, superhero film.
Invisibility is one of those superpowers that demand a little more creativity. That’s all. You can kill ’em.