This is inspired by my work on The Dolorous Stroke, but adapted for OSR games.
Here's a system for when two characters duel.
Duelling requires that two combatants agree to one-on-one combat, and are not interfered with by any third fighter. Once the duel has begun (which might be in the midst of a larger fight), the combatants continue to fight round-by-round as normal, until the duel ends. The duel might end because one duellist is slain, because a third party interferes, or because one duellist admits defeat or tries to break away from the fight.
The fight is resolved normally, with both sides rolling to hit each round on their action.
At the start of each round, however, both sides must choose their tactic for that round; either Push, Parry or Feint. Each chooses secretly and reveals simultaneously, like for 'rock-paper-scissors'. Compare each fighter's tactics, which will modify the rolls for the round.
Parry beats Push, as the parrying character turns away the obvious attacks of the push. The character who parried gets +3 AC.
Push beats Feint, as the aggressive push forward batters through the attempts at finesse. The character who pushed does +3 damage if they hit.
Feint beats Parry, as the feinting characters creates openings to strike past their enemy's guard. The character who feinted gets +3 to hit.
If both sides Parry, then the duel becomes a defensive stand-off. Both characters get +3 AC.
If both sides Push, then they just batter at each other furiously for the round. Both characters do +3 damage if they hit.
If both sides Feint, then the duel becomes a complex dance of strikes and counters. Both characters get +3 to hit.
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Feint/Parry/Push are named for sword-vs-sword duels, but you can use the same mechanic for other 1-on-1 fights. For example:
A joust has Evasive (+3 AC), Aim for the Body (+3 damage), and Aim for the Shield (+3 to hit).
A firefight with both parties shooting from in cover has Hunker Down (+3 AC), Shoot Recklessly (+3 damage), and Aim Carefully (+3 to hit).
A fistfight has Back Up (+3 AC), Swing Wildly (+3 damage) and Dirty Blow (+3 to hit).
And so on.
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What's my reasoning? I wanted 1-on-1 fights to have an extra layer of tactics to them, rather than just being 'roll to hit' each round. In my experience reenacting and larping, a 1-on-1 duel is a fairly tense affair, with both sides trying to read the other so they can take advantage of their mistakes. So, the rock-paper-scissors model works well here: predict what your opponent wants to do, and you can capitalize on that.
For predicting your opponent to be viable, there needs to be some asymmetry between the 3 options, otherwise which option is picked will be basically a 1-in-3 random chance. So, each option gives a different advantage if it succeeds, motivating you to pick different options depending on the situation. In a round, the duelist must decide if they care most about not getting hit, about damage if they hit, or about successfully landing a blow at all. If you're losing badly, pick Parry to go on the defensive, for example. Since different combatants will have different talents and priorities, you can take a good guess at your opponent's tactic based off that.
Likewise, it's possible that the situation can make one tactic obvious for you: perhaps your down to 1HP and any hit will drop you. The obvious move here is to parry, for the AC bonus, but your opponent knows that and so will want to go for a feint in order to beat your parry and get that last hit in: if you instead go for a push, you catch them off guard, negating their feint entirely and getting extra damage in.