I've read quite a few modern era war novels (Tom Clancy books), quite a few sci-fi and post-apocalyptic books (Deathlands) and written some post-apoc stories not uploaded here.
I've found that when I am forced to write a fight scene, which I tend to avoid because I wrote too many early on in my fanfic writing, I have to keep in mind that the MC can only experience and do so much. As an example, the protagonist isn't going to hear Bob yelling a warning a few feet away.
They hear nothing but a burst from their high powered rifle that they're shooting into a ruined hotel on the opposite side of an overgrown three lane road. They see Bob dive behind cover out of the corner of their eye as enemy bullets skip off the concrete sidewalk between the two people. Followed by hearing the echo of an enemy combat rifle return fire. The protagonist ducks behind a burnt car on their left, hoping whomever fired the shots didn't have an angle on them any longer.
Someone yells over the ringing in their ears, "Second floor, fifth window on the right." More gunfire reverberates off the steel canyon as the rest of the merc squad fires in that direction, filling the air with the stench of cordite and dust.
Bullets thump into the fender the protagonist is hiding behind and tear through the thin sheet metal, causing them to scoot toward the boot. They were told by their employer that the ruined hotel was empty and the supplies were theirs for the taking. Clearly that isn't the case! Even when they peer over the car's boot and look to the correct window. They only see dust and shadows until another muzzle flash appears in a completely different window on the third floor.
And a small dark shape flies right toward our protagonist for a split second. Something kicks off their helmet with enough force it feels like they were punched in the head. They dive to the ground as other bullets thump into the boot. While gunfire echoes around them, the protagonist feels the impact point on their head. Was it being shot in the helmet or was it them shooting the enemy that caused the ringing in their ears? The protagonist doesn't know. They were shot. They're alive, right? Maybe. The world feels weird as their heart races and a strange tingling feeling rushes through their hands. The once lush grass growing from cracks in the concrete at their feet looks more muted than usual. They can't feel their fingers when they check their head again.
As they sit up behind the rusty car's rear door and reach for the helmet strap, someone places a hand on the protagonist's shoulder, distracting them. The armored person's mouth is moving, but the words are distant, coming from somewhere far, far away. "Are you alright?!" the voice squeaks.
What is happening with Bob during all this? ?
The protagonist isn't going to know (hopefully you, the author, does) until they turn their head and see Bob is failing to slide a fresh magazine into his old trusty rifle, wide-eyes locked right on them, as the magazine falls from his hand.
It silently clatters to the ground as someone else yells, "Grenade out!"
Hopefully, this made sense and I apologize if it's not allowed. I tried to convey a group of four shooting at a group of three hunkered down in a wasteland hotel.
I likewise apologize for how long this became. I really meant to just say that if you do include a combat/battle scene (and not just skip to the aftermath. That's an option, too), describe things from the MC's perspective; what they see, hear, taste, smell, and feel. And make sure the reader knows what's at stake. Don't forget that commanders will only get a clear picture if communications are working. Did they break down and units now need to send runners back to command to receive new orders? Did the runner get killed along the way or are they sitting outside the mess tent eating ice cream? Did the pigeon get shot down?
If you've ever watched a YouTube video for a game like Arma, Squad & Squad 44, BF1, Verdun, Hell Let Loose, then you will see that larger combat can be quite hectic and most of it is smaller sections fighting each other.
Again, I apologize for how long this is.
TL;DR,
Don't get bogged down trying to cover every minute detail of the combat encounter. Don't forget downtime if it's a larger battle which can last hours/days/weeks/months. And don't forget to use your five senses; eyes, ears, taste, touch, smell, and character emotions. Ask yourself why the fight is happening and if it's actually needed or can be bypassed. What do we learn through the fight? Does the protagonist become injured in it or die / lose someone they know?
Combined arms is a lie~. It is bery chaotic. Nobody has any idea what is going on, neither the people on the ground nor the commanders above. Everyone tries to pretend they know something.
