One roll fights

Combat takes too long sometimes. I like the danger of it, don't like how you immediately leap into initiative and fight it out tactically blow-by-blow. It breaks the flow.
Here's an alternate system, based on a comment I read on /tg/ and some musings I've been musing for a while.

Ignore all your combat scores on your sheet. We aren't using that stuff. Instead:
First, work out the 'combat power' of each side.

For players:

• d6
• +1 if you outnumber the enemy at all. Or +2 if you outnumber them 2-to-1, +3 if you outnumber them 3-to-1, etc.
• +1 per fighter-classed PC properly equipped on your side.
• +1 per PC with one or more combat-relevant magic items (such as magic swords)
• +1 for each spell caster PC who casts a combat-relevant spell (such as a fireball) during the fight.
• +1 for each of the following advantages your side has: cover, high ground, formation that can't be flanked, surprise, superior firepower, poison, monsters are cornered, traps, monsters don't care about killing you, monsters can't meaningfully threaten you. Or similar.

For monsters:

• d6
• plus the hit-dice of the highest hit-diced monster present.
• +1 if they outnumber you at all. Or +2 if they outnumber you 2-to-1, etc
• +1 for each of the following advantages their side has: cover, high ground, formation that can't be flanked, surprise, superior firepower, traps, fighting in their own lair, regeneration or replenishing numbers, fearless, instant-death or otherwise OP attacks (turn to stone, dominate, baleful polymorph, paralysis on touch), PCs are cornered, fighting in conditions the PCs aren't adapted to but the monsters are (underwater, pitch darkness, narrow crawls), immune or highly resistant to the PCs tactics (such as resistant to non-silver weapons, etherealness, etc). Or similar.

The side that rolls highest wins. Ties are treated as a draw.
(note, the D6 roll adds uncertainty, but mostly the flat modifiers are enough that you can reliably guess who should win, and often the result is a foregone conclusion).

If the PCs win:

• The monsters are defeated: slain by the PCs or taken prisoner, whatever makes sense. 
• If the PCs lack any means to meaningfully harm the monsters (such as silver vs werewolves), the monster is instead driven off and might return.
• Each PC makes a Save vs Combat. Unarmored PCs use their worst save, armored use their best. If passed, the PC is fine. If failed, they take exactly as much damage as the d20 rolled. 
• Weird consequences other than damage (infection, level drain, curses, turning to stone) requires a second combat save to avoid it: instead of armour, you get to use your best save if you used an appropriate countermeasure (a mirror vs medusa gaze, etc), otherwise your worst.

On a draw:

• The monsters are driven off but can return, and the PCs likewise are forced to withdraw.
• Each PC makes a Save vs Combat. Unarmored PCs use their worst save, armored use their best. If passed, the PC is fine. If failed, they take exactly as much damage as the d20 rolled.
• Weird consequences other than damage (infection, level drain, curses, turning to stone) requires a second combat save to avoid it: instead of armor, you get to use your best save if you used an appropriate countermeasure (a mirror vs medusa gaze, etc), otherwise your worst.

If the PCs lose:

• Each PC makes a Save vs Combat. Unarmored PCs use their worst save, armored use their best. If passed the PC escapes, if failed they are killed.
• Surviving PCs flee or are driven off and take d20 damage in the process. 
• Weird consequences other than damage (infection, level drain, curses, turning to stone) requires a second combat save to avoid it: instead of armor, you get to use your best save if you used an appropriate countermeasure (a mirror vs medusa gaze, etc), otherwise your worst.

Example fight 1:
Our party (Black Alice the Magic User, Ser Tristam the Fighter armed with Blackrazor the magic sword, Sister Paula the cleric and Desperate Dave the thief with a pair of loaded pistols) encounters a pair of zombies as wandering monsters. Paula casts Turn Undead, Alice casts magic missile, and both Paula and Tristram block the corridor to protect the squishies behind them. 

The party get d6+8 combat strength (Outnumber 2:1. 1 fighter. 1 magic weapon. 2 spells cast. 2 tactical advantages - unflankable and superior firepower). They roll a total of 11.
The monsters get d6+3 combat strength (2 HD, 1 tactical advantage - fearless). They roll a total of 5.
The PCs win. The zombies are killed and their bodies burned.
Everybody rolls their Combat saves. For Alice and Dave who are unarmoured, this is their worst saves - 16 and 15 respectively. Tristram and Paula are armoured, so they get their best saves - 12 and 10.
Alice rolls 17, and is fine.
Dave rolls 13, and takes 13 damage - he's slain in the fight.

Tristram rolls 13 and is fine.

Paula rolls 4, and is hurt but survives.

Example fight 2:
Our party (Black Alice the Magi User, Ser Tristram the Fighter armed with Blackrazor the magic sword, and Sister Paula the cleric) are camping when their camp is attacked by a swarm of 9 wraiths! They struggle out of their tents, but Alice has no prepared spells, and Tristram isn't in his armour, although he still has his sword. Paula, since she was on watch, is armoured and casts Turn Undead again.

The party get d6+3 combat strength (1 fighter. 1 magic weapon. 1 spell. 1 tactical advantage - cover). They roll 9!
The monsters get d6+12 combat strength (outnumber 3-1. 4 hit-dice. 5 tactical advantages - fearless, fighting in the dark, ethereal, surprise, level-drain). They roll 14!

The PCs are defeated and flee their camp.
Alice fails her first combat save and is instantly slain.
Tristram passes his first combat save, meaning he merely takes d20 damage. He rolls a 17, only has 14 HP, and dies as well.
Paula passes her first combat save because she has armor on! She takes a mere d20 damage, rolls a 3 and somehow still survives! She then fails her second combat save (against the level drain - although her holy icon was brandished hopefully, even her best save was not enough to save her). She takes level drain, reducing her to level 2.
Alice and Tristram  never escape the camp. Paula is now fleeing blindly into the woods, having suffered level drain and badly injured. Maybe she will survive until sunrise. Maybe.

And, there you go. Fast, brutal combat resolved in a single round, without fiddly shit. Mostly the winner is determined by tactical advantages: if you want to win, first outmaneuvre your foe. No idea how this would actually go down in play, but I want to try it.




Edit 1: I think I want to put a gap in between 'calculate which side wins the fight' and 'see if you die'. This lets you a) cast a spell or use an item or panic and do something smart to swing a losing fight to a draw or a win and b) if you're losing cast a get-out-of-jail-free card like teleportation away (which I might allow you to save as if it was a draw if you had a free escape).

Edit 2: Obviously, this is intended as a framework to hang stuff off with room for improv and special rules for monsters, and PCs doing cool shit for bonuses and stuff, not a rigid flowchart.

Edit 3: that said 'well, you did a fight, and the dice say you die' is kind of intentional. Fast and brutal. Don't fight if you can't run the numbers and be confident of a win.