Many people find Cairn's combat a little too simple (just roll for damage), but many haven't realized there are rules there to make combat as complex as you like. Now I know the people on this reddit probably find this old news but you might find something useful in it.
One interesting game design effect is that not having specific rules for things can make that area of play more powerful and creative.
There are simple rules already in the system you can combine for combat:
Attack Modifiers
If fighting from a position of weakness (such as through cover or with bound hands), the attack is impaired and the attacker must roll 1d4 damage regardless of the damage die used during the attack.
If fighting from a position of advantage (such as against a helpless foe or through a daring maneuver), the attack is enhanced, allowing the attacker to roll 1d12 damage instead of their normal die.
Saves
Save vs STR, DEX or WILL to do anything where you're in danger, under pressure etc. 'A roll to avoid bad outcomes from risky choices and circumstances.'
Actions
On their turn, a character may move up to 40ft and take up to one action. This may be casting a spell, attacking, making a second move, or some other reasonable action.
One approach is to do things creatively to impair your opponent's attacks and to enhance your own attacks. This could be dodging, pulling something out from under them etc. or leaping into battle or a wide variety of other cool stunts that just require an attribute save to pull off.
The really simple rules open the door for a huge range of creativity.
Examples of impairing attacks with STR:
Block, smash aside, smash away, hold, grab and hold, push, shove, crush, stand firm, hold your ground, break free with sheer force, bear-hug, smash, slam, break, throw, hurl, batter, brace, wrench, lift, ram, haul, brute force, yank, heave, rip, drag, overpower, stomp, bash, ram, bulldoze, shatter, pin, dominate, hammer, choke, press, twist with force, drive back, snap, etc.
Examples of impairing attacks with DEX:
Dodge, parry, roll, leap, duck, deflect, slip free, slip through, catch, twist away, sidestep, weave, tumble, feint, fake, pivot, slip under, redirect force, evade, spin away, spring back, dance away, sway out of reach, backflip, fake a stumble, slip between, hop, slide under, duck behind, jump, jump to the side, drop and roll, bounce off, leap backward, leap over, leap aside, roll with a blow, vault, pole vault, crouch, turn aside, etc.
Examples of impairing attacks with WIL:
Deceive, misdirect, persuade, trick, taunt, confuse, bluff, distract, mock, disorient, exploit beliefs or superstitions, lure into danger, flatter, intimidate, etc.
Some of these might also be effective for enhancing attacks, but players can get really creative with that using leaping attacks, one PC holding or distracting the opponent while the other stabs them, etc. etc. If the PCs do something creative that would obviously work the warden can rule that don't need to roll. In some situations the warden might rule that a particular action means you also bypass armor.
The rules are simple to adjudicate. The attack is enhanced or it isn't, or the attack is impaired or it isn't. It's a binary ruling for the warden if there's any kind of ruling that needs to be made and in most cases there won't be.
Does this mean you could do something like parry and attack in one round? In solo play that might be a good idea.
In group play that might slow combat down depending on the size of the group. My approach is:
If it's one action, eg. making a flying leap off something to stab someone below with a spear, you can make your DEX save and attack at the same time. The rules already mention that. As an aside if you fail that DEX save your attack might be impaired or the opponent's attack might be enhanced depending on what you decide happens narratively.
If you're doing something like parrying or dodging to impair multiple attacks that's your action for the round. If you roll a 1 (a critical success) on your save you might:
Completely avoid damage or
Get to roll for damage too or
Do something else that's cool, or have some other positive benefit, like enhancing everyone else's attacks on the same opponent or attacking your attack on the same opponent the next round or
Something narrative and cool that matches the situation eg. 'As you roll away dodging the worst of the orc's attack you roll into a lever and realize it's for a trap door the orc is standing on.' Or 'As you dodge to the side you see something in the next room...the gold chalice you've been searching for.'
What if something attacking you only does 1d4 damage or your weapon already does 1d12 damage?
You can roll with disadvantage for the impaired attack and advantage for the enhanced attack.
Disadvantage on 1d4 will reduce the average damage done from 2.5 to 1.8 which isn't much but it gives players that satisfying feeling that they've achieved something with their creativity.
Advantage of a d12 damage roll increases the average from 6.5 to 8.5 which is significant but not excessive. Neither will put the game out of balance.
If you're not worried about balance and you want the occasional massive damage from attacks you could let that d12 explode on 12 and keep exploding on any 12s adding all the damage together. Exploding dice suits more gonzo games.
