On the other hand, I've been in a lot of simulated combat. I've been larping for, like, ten years, I've done airsoft and dabbled in reenactment. The biggest battle I've been involved in was roughly 800 on each side. Big enough that each side is divided into units with unit commanders, runners to take commands from the generals and return with situation reports, actual formations and manoeuvres.
Here are some observations on how combat tends to go down based on this live-action experience, how games tend to get it wrong, and maybe how to fix it.
Shields win fights. Going up against somebody with a big shield without one yourself is fucking difficult. Like, maybe 8 times in ten the shield-user wins.
Shield walls as a formation are rock hard in a direct fight. Stand shoulder to shoulder, shields up, weapons ready, and a frontal assault trying to take that on is going to have real problems. Unfortunately, shield walls, because of their tight formation, move slowly, get broken up by terrain like woods, and are hard to maneuver. Plus, they're super vulnerable if you can get in among them or round the back or whatever because the densely packed mass of fighters make it hard to respond once somebody's past the wall.
How to model this: make shields better. No, not '+2 AC rather than +1' better. Like a shield fundamentally changes how you fight. A shield user going full defensive against an enemy to their front is basically unhitable. In massed battles, make 'shield wall' a formation you can take that makes it hard to move but gives huge bonuses to defence.
Spears are good because they can stab you before you can stab them. Likewise, this is why swords are better than knives. Keep an enemy with a smaller weapon at bay, and they're not gonna be able to hit you without getting hit by your weapon first.
Counterpoint to this: if you get inside the reach of a long weapon, now the short weapon has a huge advantage. Get past the tip of a spear so you're stood next to the shaft and the tip is behind you, and you're spear-wielding victim is fucked unless they drop the spear and draw a shorter weapon. Same goes for knife vs greatsword etc.
How do you prevent them getting inside your reach if you've got spears? Lots of buddies. Line up a couple of ranks of spear users, get them (again) densely packed together and you get basically a fence of spikes pointing at you, and however you try to get past, you get stabbed. Pike blocks are like shield walls, but stabbier.
Again, this can in theory be countered by going around the side or getting right in among them if you're super reckless. Getting in among them is gonna be hard though, since behind the front row of spear-tips there's gonna be another row from the second rank, etc etc. Actually charging a pike block from the front is basically just voluntarilly kebabing yourself.
How to model this: Work out the range you're fighting at: the person who's weapon's range you're fighting at gets the first attack, and the other only gets to attack at all if they can get past it somehow.
Everybody hates archers.
There are two ways you can use them. A unit of archers all in one go can shoot at one of the dense formations above, just pumping arrows into them, and the casualties will be spread about and break up the formations: this can give you the weak-spot you need to break past their defence IF you time a charge well enough.
Otherwise, you're taking potshots at vulnerable spots, picking people off when they're out of cover or a juicy target. Shooting officers if you can tell who they are is super easy. Same for medics etc.
Arrows have limited range for being able to take accurate shots in the heat of battle. A lot of experienced archers will wait until their victim gets closer to shoot: most combatants know this and keep their distance/get into cover if they see an archer taking aim at them. Having an arrow nocked and at half-draw while scanning for targets has - in the past - let me hold of five enemies when it was just me: broken terrain between us meant I could definately drop two of them before they reached me and nobody wanted to be first.
As such, a few archers can define a 'threat area' and keep enemies at bay (unless they're heavilly armoured, bonkers, etc).
You can nock an arrow, draw and take aim at a slow walking pace. Your accuracy suffers a little, but it's doable. This is useful for projecting your 'threat area'.
Keeping an arrow nocked and half-aimed zooms your focus in on the targets in front of you. The rest of your situational awareness suffers.
You cannot shoot if somebody's actually in melee range. You're dodging about or backing up or parrying, you just can't nock an arrow in that situation. Legolas isn't real, that shit isn't practical. Drop your bow and draw a melee weapon.
FOUR: Being in a shield wall.
So, there's a bunch of you. You can see in front of you. Your flanks are obscured by your comrades to either side. Hard to tell what's going on in the wider battle, mostly. If you're actually in a fight, then you've got maybe one or two enemies right in front of you that suck up all your focus. Maybe the ones to either side of them too if the press is less dense. You know how the fight with your enemies is going. You can see how your comrades to either side are doing and - if your direct opponents give you the chance - maybe intervene too. Weapon reach is a big factor here: if you've got a spear you're much better at supporting those to either side of you than if you had a sword just because you can reach further.
Outside of you, your mates to either side, the enemy in front of you, and his mates to either side, you have NO FUCKING CLUE how the rest of the line is doing while in the thick of combat. You better hope your formation is strong and disciplined enough to hold: if it breaks then your nice shield-wall or pike-block will get rolled up as the formation is disrupted.
FIVE: Line-on-line fighting.
Line-on-line fights don't last long. A couple of minutes max. The lines start out a little outside of reach. Maybe ten yards. Push forward, try to find openings, take casualties. Pull back again.
The side that's been fighting worse tends to get pushed back in the press. This is simple, really: if your being overpowered in your duel, you'll take a step or two back to protect yourself. If the guy next to you steps back, you take a step back too so you aren't stood out on your own like a wally (and thus vulnerable).So, the side that's doing worse will take surprisingly low casualties, but will lose ground in the press and generally pull back, ceding ground to the enemy. (Do this enough and the wider formation between units breaks up, letting you start doing real damage by getting behind enemy units once the unit next to it has been pushed right back).
Discipline and leaders are everything here. Morale is a huge, huge factor. After shields/spears, morale is the biggest factor. Fights are seriously won by somebody with a loud voice shouting "PUSH FORWARD ON THREE: ONE, TWO, THREE, PUSH! AGAIN! ONE, TWO, THREE, PUSH!" and the rest of the unit going with it. If it's done to you by serious, disciplined troops who don't break formation, it's terrifying.
DO NOT BREAK FORMATION. DO NOT BE A HERO. DO NOT GO OFF ON YOUR OWN. That shit will make you isolated, vulnerable and dead in quick succession.
How to represent this: fights don't pin you in place over multiple turns. You close, exchange blows, and then one side gets pushed back. They probably don't break ranks and flee, they're just ceding ground.
You can run in armour. You can be agile in it. I have climbed trees in chainmail.
The problem is that it's exhausting. You get tired way quicker. heavilly armoured fighters are way more exhausted at the end of the fight than lightly armoured ones.
How to represent this: different movement rates for different armour/encumbrance is dumb. Enforce more rests for more armour, and if they don't punish them severely. Every event I'm at, somebody is an idiot, doesn't hydrate properly in battle, and makes themself ill: don't be that person and punish your PCs if they try to ignore fatigue.
Skirmishers are great. An enemy with a big shield, heavy armour, whacking great pike, etc is weighed down by it in the long term. Light troops aren't.
Likewise, an open and flexible formation lets you maneuvre waaaay more easilly. This is good for doing things like hitting flanks, sneaking around the back, taking out behind-the-lines juicy targets (like medics, commanders, wizards, artillary, etc), exploiting weak spots.
Skirmishers dart in, hit something vulnerable, and fall back before the formation can bring itself to bear on them.
EIGHT: Trading blows.
A simple observation: its really easy to hit an enemy in a fight, it just means that they're probably going to hit you too. Trading blows like this is pretty doable and actually fairly realistic if both sides are in full heavy armour (think gothic plate, a full coat of chain, etc). Being able to get a hit in while denying your enemy the chance to hit back is key to winning fights.
Simple ways to do this: Superior reach (or getting inside their reach). Pushing aggressively with a shield. Chucking a javelin at them as you close.
The #1 best way to do this: have a buddy with you when they don't. They can't fend both of you off.
I don't play fps games so maybe this is all obvious to you, I dunno.
COVER COVER COVER. Be in cover or somebody will shoot you. If somebody is in cover, shoot at their position and they can't leave it. Now, your mates can sneak round the flank while they likewise pin you down. Or they can sprint to a better position while you keep them pinned to prevent them picking of your mates as they make a mad dash for it.
You flush people out of cover by either going round the side or chucking a grenade over it.
If you're crazy, a bunch of you can charge straight at them firing from the hip on full auto. Surprisingly effective if you're OK taking massive casualties in order to get that one bastard who's stopping you advancing.
Camouflage is important, since it's harder to pick you off if they aren't sure exactly which bit of the cover the shots are coming from. Colours that match the environment are nice (neon colours get you killed), but more important is breaking up your silhuette. Ghillie suits and capes are great for this.
TEN: Discipline and formations.
I've said this a lot, but seriously. If you break formation, you will get picked off by archers and skirmishers. If your formation loses its coherency, it loses its protection and light, aggressive troops will take advantage of it.
HOWEVER. Forming a shield wall and parking yourself somewhere will get you killed as more maneuvrable troops go round the side. Or just ignore you and go for the vulnerable targets. You need to keep moving forwards (to keep the enemy pushed back) or failing that to be able to fall back to a better position without turning your back.
Ideally you want to keep pushing forward. Hit hard, push the enemy back, claim ground.
DO NOT form a circle. If you're back-to-back with your allies, you can't step back if you need to. You'll be surrounded, unable to retreat, and pressed so dense you can't fight properly. You will all die. All of you, to a man. Better to break formation, run for it, and reform somewhere safe.
ELEVEN: People on the floor.
Executing a fallen enemy is quick and easy and you can do it as your line pushes past them. Do it to avoid nasty surprises: if you don't you are an idiot and deserve it when your enemies get mysterious reinforcements from behind you.
You'd be surprised how often you can pick up stragglers, injured allies (or ones who've lost a weapon/shield but are still in the fight) to reinforce you if your enemy aren't doing this.
TWELVE: Big monsters, elites, vehicles etc.
Something big and scary is basically worthless if it's not supported. A war-rhino or chariot or whatever will be surrounded and dispatched really easy if the enemy knows what they're doing. So, put the secret weapon in your formation, guard its flanks. If you don't, it's isolated, and isolated combatants quickly become dead combatants.
Much of this is based on my experiences at the UK larp Empire, and it's preceeding systems Odyssey and Maelstrom. If you're in the UK, you should absolutely try the events, they're pretty brilliant. There isn't much that compares to being part of a huge medieval style pitched battle in terms of sheer visceral spectacle.