an even quicker combat system

 

Unlike my other quick combat system, this doesn't stand alone and therefore isn't MOSAIC strict. Instead, you can graft it on to any other system with a task resolution system where characters have something like skill ratings, like Pottery +5 or whatever.

This represents a small skirmish in which death for PCs is a serious possibility. If you don't want the possibility of death, either change the stakes or skip the rolls and describe how the PCs win.

1. Flee! Parley! Reconsider! Speak now or forever hold your peace!
   
2. Make a relevant skill roll (or make "combat" its own skill) twice. The first represents your ATTACK, while the better of the two represents your DEFENSE. 
3. Take the median ATTACK (rounding "up" if there are an even number of PCs) of the PCs. That's their GROUP ATTACK.
4. Among the opponents of the PCs, take the Nth best ATTACK as their GROUP ATTACK, where N is the order number that gave the median order amongst PCs. (So if there are 5 PCs, you use the third best monster attack just like you used the third best PC attack, regardless of how many monsters there are.) If there are fewer than N monsters, give the lowest monster an additional ATTACK roll and take the lower as their GROUP ATTACK.
   
5. Everyone whose DEFENSE is lower than the other side's GROUP ATTACK is dead.
6. Go go back to Step 1.

To combine this with something else going on - trying to disassemble the Apocalypse Orrery whilst the others fight off laser sharks - have the PCs who are doing the skill check contribute zero to ATTACK (while they roll for relevant other things instead) while still counting as a party member for rolling DEFENSE and counting towards the median.

Example: Alice the fighter, Bob the wizard, and Carla the rogue encounter a Copyright-Nonspecific Floating Eye Creature. Alice rolls 1d20+7 = {11, 20}, Bob 1d20+3= {7, 12}, and Carla 1d20+5 = {24, 12}. The CNFEC rolls 1d20+6 = {14, 20}, and since there's no second creature he attacks with disadvantage at {{14, 13}, 20} giving us raw scores of

	Attack	Defense
Alice	11 (counts as GROUP ATTACK)	20 (> 13)
Bob	7	12 (<13)
Carla	24	24 (>13)
CNFEC	13 (counts as GROUP ATTACK)	20 (>11)

Result: wizard dies from laser beam to head.