OK so, a problem I have with a lot of the systems used to resolve violence in RPGs is that attrition is not particularly exciting. Often, each side has a pool of HP, and take it in turns to make attacks against the other, slowly wearing down that pool until one side or the other has none left, and loses.
I don't find this particularly exciting.
I'm taking inspiration from anime here, and how shonen fighting shows (or at least, the good ones) often handle fights. Essentially, each fight is a puzzle. Each side brings their own techniques and advantages, and the other side has to work out how to negate those methods to bring their own to bear. A fight tends to swing one way and then the other, as one side sets a challenge and the other has to find a solution or be defeated; once a solution is found, then the fight swings the other way until the losing side finds a solution of their own, until once side gains a decisive advantage that the other simple can't answer, and is forced into defeat.
Ultimately a fight is won by wits, creativity and adaptability; the ability to formulate a plan that your enemy can't find a counter to.
This is the dynamic I want to capture.
Here's a system to do that.
It doesn't care much about the numbers on your character sheet, and is instead driven by what's happening in the game-fiction. Negotiating and defining what's happening in the fiction is how you win.
Determine the stakes of the fight. What will happen to each side if they lose; death, capture, humiliation, injury, whatever.
Determine the capabilities of each fighter. If you're bolting this onto an existing system, this might be easy. Looking at a system like D&D 5e or VtM, a character probably has some clues as to their capabilities and powers on their sheet; just pick out what the key elements are.
If you're not bolting this onto an existing game, you can instead determine your fighter's capabilities quite simply. You get to state three advantages they have. These might be:
-A weapon they use, and its quirks.
-A supernatural power they have.
-A wildly impractical stunt or maneuver they've learned, and can pull off reliably.
-A broad fighting-style they're skilled in.
They can get two more such advantages, but for each extra one you have to state a weakness they suffer from.
Who's winning is measured on the winning-o-meter, a sliding scale from -3 to +3. When it reaches +3, one side wins, when it reaches -3 the other side wins. It starts at 0. Over the course of the fight, the winning-o-meter will go up and down depending on who's dominating.
EG: Alice wins at -3, Bob wins at +3.
Control determines who's currently setting the stakes for the fight. The character with control is the one who has set up a situation that the other must find a solution to, or be defeated.
Which character begins with control is a judgement call. It will probably be the character who's overall stronger, attacking from an advantageous position, striking from surprise, and so on.
EG: Bob started the fight, so he begins in control.
The fight is divided into Exchanges. When an Exchange begins, the player who's character is in Control gets to state a fact about the fight and why it gives them the advantage over their enemy. The character not in Control must attempt Gambles until they overcome this, which ends the Exchange.
EG: Bob might state "My spear easily out-reaches your sword, meaning you can't get close enough to hurt me while I can attack you with impunity".
To make a Gamble, the player not in Control states what their character does, and how it will overcome their enemy's advantage.
EG: Alice might state "I'm going to feint to one side and dash past your spear-tip, so I'm in sword's-reach of you and too close to easily attack with your spear."
To resolve the gambit, roll a d10. The base chance of success is 5-in-10. Circumstances may modify that base chance, but it can't get worse than 1-in-10 or better than 9-in-10.
The chance is 1 better for each of the following:
-The gamble is totally unexpected.
-The gamble takes advantage of the enemy's weaknesses.
-The gamble leverages one of the character's strengths.
-The gamble uses the environment to its advantage.
-The gamble turns the apparent strengths of the enemy's technique against them.
The chance is 1 worse for each of the following:
-The gamble was easily predictable.
-The enemy has taken counter-measures against this sort of attack.
-They've used a similar gamble already.
-The gamble is hindered by environmental factors.
-The gamble is unusually risky.
It's a judgement call which of these apply, of course.
If a gamble won't realistically be able to overcome the character in control's advantages, it can't be attempted at all.
If a Gamble Fails:
The exchange continues. The player in control gets to state another fact about the fight, and the subsequent gambles must overcome that advantage too. Further, the winning-o-meter shifts one point in favour of the player still in control.
EG: the gamble fails, so the winning-o-meter shifts from 0 to +1, and bob is 1 step closer to winning when it reaches +3. Bob then states a new fact: "not only do I have a reach advantage, but you've been knocked to the floor."
If a Gamble Succeeds:
That exchange ends. Control flips to the player whose gamble just succeeded, and the winning-o-meter shifts one point in their favour.
The facts established for the exchange so far are negated by the successful gamble, and the player newly in control starts a new exchange, stating a fact of their own and why it gives them the advantage.
EG: the gamble succeeds, so that Alice gains control, and the winning-o-meter shifts one point in her favour, from 0 to -1, bringing her one step closer to victory. A new exchange begins, and Alice states her advantage: "From my diving attack along the ground, I've cut your hamstring, hugely reducing your ability to maneuver or even stand properly".
The fight is over when the winning-o-meter reaches +3 or -3.
The player in control can, rather than stating a fact about the fight to give them an advantage, state that they've safely withdrawn from the fight.
Adjusting the Winning-O-Meter.
You can make a fight shorter and more brutal by making the winning-o-meter range from -2 to +2. Likewise a fight can be made longer and more complex by extending it to perhaps -5 to +5 or even more.
A particularly one-sided fight, perhaps where one fighter is much stronger than the other, or an ambush, might be asymmetric. Perhaps it ranges from -2 to +4, giving one side far less room to fail and the other much more wiggle-room.