d12 Oops I overcooked this encounter

So your players got lucky streak and did some clever things and creamed some baddies and now its the big boss and you wanna flex a bit so the heroes suffer a bit and feel some thrills. Then all the dice turn bad and it turns into a possible TPK...

Game balance is a bit silly
If you over do it there is no risk
It can waste lots of tims
BX did HD +1 for major magic abilities worked fine and fast

I like older warhammer and Stoembringer with more chaos and less balance. One player would be a sorcerer who could call a god and another player would be a bigger one with no legs. And that beggar would save those wizards again and again and procure the human sacrifices they needed. 

You have more options as a DM than Fudging rolls
But also dont just Deus Ex Machina too hard that players feel useless

Morale used to help with this because monsters showed more delf interest than fight to the death. What if every monster hurt took 2 or 3 soldiers off the battlefield to help them like real armies do. If your gonna fudge Morale is easier and better than combat rolls. Or Say a bos dies make a roll half die make a roll. It means lots of monsters will fall back rather than act like fanatics. Fanatics should be more scary, and berserks should be frightening. 

Its best players mostly attack first and make informed choices about attacking most of the time. Then it makes occasional mistakes and deceptions or ambushes against them stand out. It is ok to withdraw and ok to teach players this.

This is some stuff you can do in a battle to de-escalate

I like to reroll initiative when some chaotic event occurs changing the battlefield

d12 Oops I overdid this encounter 
1 Enemy has somewhere else more urgent than dying here so withdraw if not over already

2 Enemy demands adventurers surrender or talk terms like collect wounded

3 Third party or monster arrives and the original foe wants out of here fast or stops the attack. The third-party might be hostile to both or neutral or might be appealed to or bribed

4 One of the boss's personal guards turns on them and backstab the leader

5 An enemy goes berserk or turns into a lycanthrope or is really a shapeshifter and turns on their own kind closest to them. Perhaps enemy dead arise as zombies

6 Enemy hears an alarm or scream or a runner comes to fetch them urgently Perhaps a magical message reaches the leader 

7 Fire, flood, weather, swarms of vermin divide the field

8 Quake, sinkhole, cave-in or ceiling or floor collapses leaving holes and rubble and breaking up the field

9 Enemies fighting amongst themselves, kill the right leader and they withdraw

10 An enemy staggers and bursts open with dungeon parasites horrifying everyone, the enemy are demoralised and panic not even sure of each other, some busy killing parasites, utter chaos

11 Some invisible being kills some of the enemy freaking them out

12 Spirit of local area angered by enemies and offers some blessings, healing or carries away wounded to safety
  

d12 Oops I undercooked this encounter 
1 Reinforcements trickle in to replace fallen

2 Some creatures have poison weapons or missiles

3 Is it a Trap? monsters retreat luring the enemy into a hazard or for a new group to flank the adventurers 

4 A second group of enemies or friendly monster joins their allies

5 Some enemy goes berserk and are fearless killer maniacs

6 Some old enemies of heroes have sent assassins or a magician to aid party new enemy - both sides agree to divide adventurers after the battle

7 Enemy has a grenade or fireball charm or other damaging limited number of uses weapon

8 Some of the enemies are not what they seem -  lycanthrope or other shapeshifters takes the lead against adventurers  

9 Several have spells or magic items with one use like healing, summoning, a spell effect

10 An enemy has a spirit trapped in a bottle they throw to release possibly a poltergeist or something far worse the server who released it from the bottle. They aid the enemy vs adventurers

11 Several of the enemy are spell casters blending in to not be targeted, they stop holding back and use spells 

12 Turns out the boss of the group is some kind of hero with own back story, extra levels and a magic weapon