How to write "smart" characters without just having everyone say that they're smart?

Was planning out my potential series, and a later antagonist was supposed to be kind of scrawny but manipulative and smart.
It's like a whole brains versus brawns theme, because the MC is having his own little villain arc and he's getting really violent.
Thing is, MC isn't stupid, he's like a little above average.
How do I write smart people when I am very much not.

_{Not me changing my Oneshot's plot because HOW IS IT TAKING ME A WEEK TO MAKE ZERO PROGRESS}

Thank you! I'll take what you've into consideration, for sure.
Took me until now to realize that there are different types of smart.
(ex. Book-smart without common sense, Street-smart but a F- average in school, etc.)
Also, yeah- Internet access would solve a lot of these dweebs problems, wouldn't they- 😅

Ok so, yes, murder almost all my characters? Or, no, don’t kill everyone on a sleep deprived whim?

Maybe-
Depends on how and why. Like is it to prove just how big a threat is? Sure. Is it to push the MC into a breaking point? Sure. Is it an ulitmate sacrifice during the Final Boss Battle? Sure.
Pretty much any reason that isn't solely shock value.

Realizing how much alliterations and shit I be writing- can't wait for some bored English teacher to hyper-analyze every single one.

Pater marched through the snowy steams, silent save for his booted footfalls.
How long had it been? Seven seasons, was it? For seven long and lonesome seasons, Pater hadn't been home.

Ok so, yes, murder almost all my characters? Or, no, don’t kill everyone on a sleep deprived whim?

Maybe-
Depends on how and why. Like is it to prove just how big a threat is? Sure. Is it to push the MC into a breaking point? Sure. Is it an ulitmate sacrifice during the Final Boss Battle? Sure.
Pretty much any reason that isn't solely shock value.

Spoiler - click to show.

Was planning on making a likable character, and then I'd kill them because of the MC's actions. It's shock value that serves a purpose- it's supposed to push MC to a breaking point/realization.

(sry i thought i replid earlier)
the first major character death is what makes the MC snap and the whole group separate. the second time two die at once in the big boss battle to prove how far gone the villain/MC is. but in the books after i don't know what to do with most of my characters, the main group was really big and im not quite sure what to do with them.

Maybe have some of them be retired heros? Or just generally not as important?
They don't have to die for that to happen, just have them shopping for bread or something and some kids in the supermarket go,
"Heyy, didn't you save the world or something? Neat. Wanna vape?"
Like maybe a newer character sees one of the older heroes just vacuuming or something, and the newer character is like,
"oh my goodness give me your autograph PLEASE."

^{Been thinking about .}

Just thought of it on a whim, really, but I thought it was neat, probably.
Keep in mind this is a subplot, they're mostly doing other stuff. Also huge spoiler warning, if ya care.

Been considering keeping a character in my series after their death. It'd give a POV of the series' afterlife, and also a sort of spirit guide for the remaining main characters.
Main issue is the character's death was also supposed to, y'know, be emotional because it's death.
Not sure if keeping the character around and conscious would cheapen the moment because they're dead, yeah, but they're still there.
Maybe I compromise and they only show up as hallucinations?

Do I give the MC a villain arc? I need an excuse for him to be an asshole again.

Anger over a death can cause a villain arc, till MC works out his grief. So yes? If you need him to be a jerk?

Oohhh, literally perfect
guess who's gonna become a degenerate for like, 80% of the third book-

Coming up with some subtitles for my series and I'm having fun with it. Essentially it's going to be two main series with four books each.

First Arc:

First arc has a lot to do with fire. MC's backstory? Fire. MC's best friend? Fire. Main antagonists? Fire. Wanted to include fire in some way in all the titles.

Book One: From Fire
Book Two: Kindling Coals
Book Three: Flames Within
Book Four: Snowing Cinders

Second Arc:

Second arc has more to do with the in-universe idea of the afterlife. Essentially, if you die a hero, you live in the sky as a star, and you spend the day huddled with other stars to form the Sun. If you die a villain, you go to a place called the Nevermourn _{^{(i.e., hell)}}.

Book Five: Among the Stars
Book Six: Nevermourn's Talons
Book Seven: Moonless Night
Book Eight: Daylight's Graces

Assuming I have the motivation to do anything for this series that isn't just worldbuilding, I'm pretty hyped for this little project! Might plan a third series, but I was heavily considering killing MC at the end of Daylight's Graces. Not fully decided on that yet-

Weirdest things you've researched while worldbuilding / plot-planning / etc? Here's a few of mine:

• Can bear-traps break human bones?
• How much blood is in a human?
• How long does it take for a child to bleed out?
• How long can you survive on raw meat?
• How to survive an avalanche?

Ok- what prompted you to want the average density of a gorilla?