The rules of a roleplaying game should support those activities that will involve players the most, and like it or not, most time in RPGs (especially the computer variety) is spent in combat. Which also just so happens to be the most in need of rules.
How to handle combat in a game depends on your priorities. Battles&Balances focuses on making sure each enemy provides the right amount of challenge to players of a certain power level. Another goal is to ensure that each combatant and each fight feels different. You can have glass cannons, that hit hard but go down easily; slow ponderous fighters that can both take and dish out a lot of damage, if only they catch you. Quick, light skirmishers that deliver a flurry of weak attacks; snipers that strike from afar but can't run.
Of course, gear matters too, and luck can be tilted one way or the other.
Battles&Balances doesn't have a concept of being "in combat" versus "out of combat". That's just a general term for when characters repeatedly attack each other until one side is defeated and out of play. What counts is whether the combatants are in melee range, missile range, or out of range entirely.
Key to this combat system is that a character's attribute scores represent a number of dice. To see how well the character succeeds at attempting a task involving, say, muscle you roll that many dice of the character's size. E.g. for an average human with a muscle score of 3 you would roll three eight-sided dice, 3d8 in RPG parlance, and add up the results. If any bonuses or penalties apply, add or subtract them from this total. This is called "rolling muscle".
So for one character to attack another in melee, first roll agility. If they have a weapon equipped, also add its attack score. The defender in turn rolls speed and adds their shield's defense score, if any. If the attacker is smaller than the defender, half the difference between their respective sizes is added to the attack roll. Otherwise if the defender is smaller, the same is added to their defense instead.
Either way, if the attacker's grand total is higher, they hit and get to deal damage. Otherwise, they miss.
Damage is calculated much the same way: roll the weapon's damage die, add or deduct the attacker's damage modifier, if any, and finally the margin of success: by how much the attacker's roll managed to beat the defender's. Then, if the defender is wearing any armor and the damage is at least equal to the armor's protection score, reduce the damage by that much.
(The latter rule simulates the fact that being clubbed is going to rattle your insides even through full plate. But in game terms it's to avoid the situation where evenly matched opponents keep hitting each other but causing no harm.)
After all that, if there's any damage left above zero, add it to the defender's wound score. If a character's wounds ever equal or exceed their health, they're defeated and out of play. As mentioned in chapter 2, that doesn't have to mean dead, but still unable to keep fighting for the moment.
Those are the basics. For ranged combat, the attacker simply rolls focus instead of agility, and the damage die comes from the ammo, not the weapon. The damage modifier doesn't apply, but everything else works the same.
Let's bring back one of our characters from chapter 2, and deck them out:
**Prince Vasyl of Ostgrund**
Human (d8)
Muscle: 4, Stamina: 2, Agility: 4, Speed: 2, Focus: 3
Weapon: Hardlight sword, +5 attack / 1-6 damage
Shield: Hardlight kite, +5 defense
Armor: liquid metal, +5 protection
Attack roll: 4d8+5 Damage modifier: +4
Defense roll: 2d8+8 Health: 16
Ready? Now let's bring in a worthy enemy for the prince to fight:
**Shock Trooper of the Star Republic**
Human in power armor (d12)
Muscle: 5, Stamina: 4, Agility: 2, Speed: 2, Focus: 2
Attack roll: 2d12 Damage modifier: +12
Defense roll: 2d12 Health: 48
The shock trooper has just been disarmed, but thanks to power armor they fight like a much bigger creature. Which just so happens to give our hero an extra edge: he gets a +2 bonus, or (12 - 8) / 2, to both attack and defense.
Now for the actual fight. An overconfident opponent allows the prince to strike first. He rolls a 1, 7, 8 and 3, then adds 5+2=7 for a grand total of 26. The enemy soldier can't possibly roll that much. He manages a 12 and a 4 for a total of 16. Vasyl's margin of success is 10; he adds his damage modifier of 4 and rolls the sword's 1d6 for another 5, dealing a staggering 19 damage!
But his power-armored opponent still has plenty of fight left. They roll a 9 and a 7 for attack. The prince only manages a 6 and a 2, but adds another 5+2. That barely fails to block the attack, and the shock trooper deals 16-15+12=13 points of damage! Even with his armor absorbing 5 of them, the prince still takes 8 wounds, cutting his health in half. Another hit like that and he's beaten...
All this illustrates not just how the rules play out in practice, but also how you can reinterpret unusual characters in terms of the same rules.
So far we've allowed each character to take one action in turn, as if they all moved equally fast, but that's kind of silly when we make a point of tracking their respective speeds.
To fix that, first give each character a number of action points (abbreviated AP), equal to their speed score times size. In our example above, the prince would get 2x8=16, and his opponent 2x12=24. Then have them take turns as before, except now they spend a number of APs every time. If a character is out of APs when it's their turn, they can't do anything, but their APs refresh. And here's the real trick: if they have even one action point left, they can always act, borrowing the difference from the next round!
(Where "round" is meant in a loose sense. Since characters will have their APs refreshed at different times, there won't be distinct game rounds to speak of.)
Question is how many APs each action takes. To draw a weapon, it could be as many as its weight. To move out of an opponent's reach, it could take the latter's size in APs. And to attack, spend a number of action points equal to the attacker's size minus three. In other words, Prince Vasyl would use 5 every time, and the shock trooper 9 -- nearly double. Surprisingly, that only makes them a little slower overall.
The above rules involve a fair amount of number crunching. If for some reason you need to cut down on the dice rolling, simply ignore speed and shields and have both sides roll agility. Whichever of them rolls higher, if any, deals damage to the other according to the usual rules. Repeat as needed. (This is best used with settings where people duel with rapiers and swordcanes.)
To simplify even more, don't bother rolling the weapon's damage die, and simply deal maximum damage every time. The margin of success makes for enough variety by itself. This makes combat brief and deadly, which is also a good fit for more cinematic settings. You might want to stick with lighter weapons, or else use armor with a higher protection score.
Speaking of the damage die, you can account for weaknesses and resistances by doubling or halving it as appropriate. E.g. if you hit an undead enemy with a silver blade, roll 1d12 instead of 1d6, whereas if you hit a rock elemental with the same, roll 1d3. Since the latter can't be found in real life, if playing on a tabletop just roll as normal and divide by two, rounding up.
Moreover, some weapons may cause persistent effects such as poisoning whoever they hit or setting them on fire. It's tempting to make these effects deal damage over time, but that just delays a death spiral, as the afflicted character likely can't deal with it right away. Better to do it like this: the first time a successful hit causes such an effect (on a coin flip for instance), write it down and deal 1 extra damage. If it happens again, put a checkmark next to it and deal 2 extra damage. Repeat as needed. The counter resets to zero once the effect is cured.
Either way, it may seem strange that armor can take blow after blow and never suffer from it. For more realism, every time a character's armor absorbs damage, have it take half that damage itself as wearout. Once its wearout equals or exceeds its weight, the armor becomes unusable. As armor only absorbs damage over a certain threshold, it will last longer than you might expect, unless the game involves lots and lots of combat.
If you use this rule, make sure armor protection scores are always even numbers. And of course it conflicts with the optional piecemeal armor rule as described in chapter 3.
The rules so far are designed to smooth out randomness and ensure combat is fair in the long run, but not too predictable. In particular, you can always see when a fight isn't going in your favor, giving you a chance to retreat. But sometimes that's not an option, and at other times the dice will just kill you by surprise.
To keep that from becoming too much of a problem, you can grant player characters a blessing every time they take damage (perhaps over a certain threshold, say half their size). Each blessing simply means that on the next turn they're allowed to roll twice for attack (or defense) and keep the better result: a so-called blessed roll. Spells, or a cleric-type party member, could be another source of blessings.
Conversely, certain enemies such as undead could deal curses instead. These work in reverse, forcing you to keep the worse roll, which makes things more difficult. A weapon and/or shield can also be permanently blessed or cursed, making all attack or defense rolls, respectively, work this way. But that can unbalance combat, so be sure to take it into account.
Naturally, none of that applies to armor. But it may well apply to rolls outside of combat.
Martial as they may be, sooner or later RPG characters will also need to test their abilities in peaceful situations. Fortunately, that's a lot simpler than what we've seen so far, if otherwise similar.
Let's take another example: Baerg the fighter is at the tavern waiting for his drink, when a huge orc challenges him to arm wrestling! The orc is a d10 creature with 4 dice in muscle, but while Baerg is only a d8 human, he's got 5 dice in the same attribute! The orc has never met a human so big and strong before.
To simulate the contest, roll the dice three times for each side, and compare the totals, either on a best-two-out-of-three basis, or more simply by adding up all the rolls and only comparing at the end. And while the two seem evenly matched, Baerg has a slight edge due to more dice. The orc is likely in for a surprise...
But not all contests are between two active opponents. To bring back another character from chapter 2, let's have Jinx the Thief on the run from the city guard, with only a few seconds to jump over a tall fence, lest they be caught. It makes no sense to roll dice for the fence, since an inanimate obstacle by definition just sits there. Jinx still rolls three times, but all they need to do is reach or beat a set target number.
Choosing the latter is an art, but the following thresholds should work well: 10 for trivial, 20 for easy, 30 for routine, 40 for challenging, 50 for difficult, 60 for unlikely, 70 for impossible. This allows even a tiny, crippled character rolling 1d4 to succeed at a trivial task, while a healthy human rolling 3d8 can still fail the same. Jinx does rather better, rolling 14, 16 and 17 on their 4d6 in agility -- enough to succeed at a challenging task.
(Then again, notice how even an average human rolling 3d8 can in fact succeed at an "impossible" task, while a 6d8 demigod has a fifty-fifty chance.)
Of course, Jinx only gets one try here, for obvious reasons. To limit how many attempts a character can make when time isn't an issue, you could impose a penalty for failure, such as taking damage or having an item break. And where that doesn't apply either, there's simply no point in rolling the dice.
Either way, sooner or later characters will succumb to their wounds unless you give them a chance to heal. Simply using food for that, like in certain modern MMORPGs, works surprisingly well, even if it's not at all realistic. Another common option is rest, but that only works if:
Otherwise, you're better off granting players an opportunity to heal at key points in the game, much like in the case of experience -- either by granting access to a healer (if the game's fiction supports it), or else simply ruling that they remove a certain amount of wounds. And speaking of XP, a character increasing their stamina will have more health, which makes the same amount of wounds less serious. But they still add up, so healing remains essential.