2D6 COMBAT SYSTEM FOR OD&D AND SIMILAR RETROCLONES
NOTE: this system assumes using d6 for hit points, HD similar progression similar to the LBB. In my games name level is 7 and the maximum level for all characters. This also assumes that ability modifiers are limited to +1 for over 14.
Determine encounter distance (3d6x10 feet or yards, or determine based on terrain and dungeon layout)
Check for surprise (1 or 2 on d6) - if one side is surprised the other gets a free round.
Determine monster reactions (per whatever rule set you are using)
Determine party actions: flee, combat, parlay, or wait for monster
Roll initiative for each side if combat or flee (each side, highest on 1d6)
Declare spells in order of initiative
Highest side does movement and resolve missile combat
Resolve Melee combat from that move and retreats
Next highest does movement and resolve missile combat
Resolve Melee combat from that move that is new or from retreats
After all moves, missile, and Melee resolve spells in initiative order (use whatever rules regarding you normally use if a magic user or cleric is injured during combat or see rules below for hit while spell casting)
Repeat steps 5-12 for new round if one side unless one side gives quarter, flee, or is dead.
Note it is possible for a character to fire a missile, charge an opponent, and enter Melee depending on the circumstances. A DM for example may allow for a character to throw a dagger while charging in battle, but may not allow someone to fire a bow and charge and pull out a sheathed sword. Use common sense and what is allowed at your table for norms to determine what is possible. If in doubt ask the DM, and remind him of your awesomeness.
All characters and monsters have a new stat, Combat Prowess (if you don't like this name then give it a different name). For monsters their base combat prowess is their hit dice. Normal humans have a combat prowess of 0. Fighters have a combat prowess of 2 and gain additional +1 each additional two levels ( 2 at level 1, 3 at level 3, 4 at level 5, etc.). Clerics start with a combat prowess of 2 and gain additional +1 each additional three levels (2 at level 1, 3 at level 4, 4 at level 7, etc.) Magic-users start with a combat prowess of 1 and gain additional +1 each additional three levels (1 at level 1, 2 at level 4, 3 at level 7, etc.) Elves or their equivalent depend on what rule set. If you are using something like a basic D&D clone then treat them as fighters. If you are using something more like the '74 OD&D rule set that forces elves or their equivalent to choose how they are treated before each adventure then use ever fighter or magic-user.
Additional Modifiers for Combat Prowess:
Fighters (and their racial class kin) get +1 for strength of 14 or greater for resolving Melee combat
Anyone with a dexterity greater than 14 get +1 for resolving missile combat
Anyone with a shield get +1 for resolving missile combat when they are a target.
Anyone with a two handed sword, halberd, two handed battle axe or similar heavy weapon gets a +1 against armored foes
Any other situational Modifiers that the DM warrants (ranging from -2 to +2) allowing players to go hog wild to show their awesomeness in the situation.
Instead of a to hit roll, each of the combatants rolls 2D6 to see if they are hit. If the roll is equal to or greater than 8 then they are not hit that round from that combatant. A roll of 12 is always a miss or a critical fumble for the other combatant, and a roll of 2 is always a critical hit. With two combatants their are four possible outcomes: they both are not hit, they both are hit, one or the or the other is hit. If they are hit, then they take damage - usually a 1D6 or a 2D6 if a critical hit. Note some games use variable damage for weapons, if so replace the D6 with the variable damage. Some more lethal games will use an "exploding" D6 for damage (each 6 rolled allows you to roll and add to previous rolls). You resolve combat by having combatants roll at the same time for each pair of combatants, but only resolve for each pair once during the combat round. Example: Andog the bruiser is fighting Cal the dervish and Andog has a 6 initiative and Cal has a 1 initiative. Andog moves by charging into Cal swinging a broken chair leg they will both resolve their Melee at the same time by rolling to see if they are hit after Andog charges on Andog's initiative. On Cal's initiative if he chooses not to flee you will not roll again to see if they are hit since that Melee has already been resolved that round for that pair of combatants. The advantage of initiative is to setting up who is fighting who.
Missile combat works the same way, but only the target rolls to see if they are hit. If two combatants are shooting each other across the battlefield they would each make their roll to see if they were hit on their own individual initiative.
Each pair of combatants will roll each round. If you have a dog pile of people fighting one character then that one person will roll to see if he was hit for each one in the dogpile and each character in the dog pile will roll to see if they are hit. Example Elruk the King is confronted by three Frwen Assassins. The Assassins have the initiative and they all do spinning double forward flips to surround the King. Elruk would roll three times to see if he was hit, and each Assassin would roll once to see if they are hit. If the Assassins all had different combat prowess, Elruk would factor his Modifiers separately for each roll (see below).
Some badass monsters have multiple attacks. This works just like multiple combatants. The character makes a roll to see if they are hit for each attack the monster has. If a monster has two or more weak ass attacks (e.g. D4 or less damage) it is suggested to just replace with one D6 attack.
The base roll to see if a character or monster is hit is 8 modified by the following:
The one with the highest combat prowess (recall that with modifiers Melee and missile combat prowess can be different) gets a +1 to the roll, the one with the lowest gets a -1 to the roll.
If the combat prowess is ludicrously different say more than 5 apart, the modifier can +2 for the higher one and -2 for the lowest one.
Leather Armor grants a +1 to the roll, chain +2, plate +3. Generally for conversions each 2 increments of AC (ignore shields see below) grant +1. Example monsters with an AC of 5 would get a +2, one with an AC 1 would get a +3, one with AC -1 would get +4.
Magic weapons generally cut or batter through armor like butter, they reduce the armor bonus of the opponent to zero.
Any other situational Modifiers that the DM warrants (ranging from -2 to +2) allowing players to go hog wild to show their awesomeness in the situation. Note a +1 or -1 modifier is a big deal for a 2D6 roll.
With a to be hit roll and combat prowess players have two aspects to modify based on the situation. In general use your judgment to determine what makes sense, recognizing that small benefits (like higher ground or hazardous terrain) should impact combat prowess, while big benefits like leaping down spear first into a horde of goblins from a cliff above should impact the to be hit roll. Also recognize that anything that makes you more of a badass should be a positive modifier to combat prowess, while anything that makes you easier to hit such as a being blinded should be a negative to a to be hit roll but something that makes it harder to be hit such as a standing behind a parapet looking down on the horde of archers should be a positive modifier to a to be hit roll.
Shields grant a +1 to combat prowess against missile attacks. Also once per encounter you can reroll one of the two dice in your to be hit roll. Magic shields may let you reroll twice or thrice. Rings of protection and similar magic items work the same way.
For a fighting retreat from a Melee you make a to be hit roll, but your opponent does not. If you break and flee you make a to be hit roll but your combat prowess is 0.