The aim of combat in 6d6 RPG is not to kill the characters. A Game Leader can do this any time they want by throwing wave after wave of over-powered creatures at the party. Instead, fights should be like a roller coaster: full of ups-and-downs, fear and screaming but with everyone arriving safely at the end, laughing about the experience.
The art of being a good Game Leader is to push the characters to the edge of their abilities but no further. To achieve this the creatures will have to pull their blows occasionally by choosing a sub-optimum combination of cards to attack with or by not targeting the most vulnerable character. Having a creature simply taking a more defensive strategy for a round or two, refilling their pool or switching cards, will give the characters a chance to recover.
With the randomness inherent in the game, with dice rolls and the initiative system, sometimes characters will be knocked unconscious and be in danger of dying regardless of what the Game Leader does. This is a good because it reinforces the idea that fights are dangerous.
Once a character is down, the creatures should ignore them until the fight is over. The character's situation is not going to become any worse for an hour (see Life & Death) which should give the rest of the players plenty of time to revive their unfortunate comrade. Even if the creatures win the fight and drive off the characters, the creatures will just strip the obvious treasure and leave the body, allowing the rest of the party to sneak back and recover the corpse.
Living creatures, whether dumb animals or evil wizards, do not blindly run into fights. They will retreat when in danger, try to out-flank enemies, gang up on easy targets and do whatever they can to win the fight. This is also true for most non-living creatures such as robots or ghosts. Only a few monsters like zombies blindly succumb to blood-lust.
The biggest danger to any creature (or character) in a fight is being outnumbered. With all the creature's cards put into defending against the first attacker, they are left exposed to the second or third attacker. Monsters who just charge headlong into a group of enemies will very quickly find themselves on the endangered species list.
Whether they bravely run away, bug out, cheese it, scarper or make a tactical withdrawal, it's all the same. Putting as much distance as possible between yourself and a stronger enemy is a good tactic. Creatures and NPCs should follow the same plan - rather than standing around being killed to the last man they should run.
This presents the characters with an interesting dilemma. Retreating creatures can regroup and attack again but they may also hole up in a highly defensible location or lead the party into an ambush. Giving chase is also likely to split the party and allow the enemy to gang up on individual characters.
When the characters find themselves in a difficult situation they should be encouraged to run away. Creatures are more likely to lick their wounds and regroup than give chase.
Avoiding combats where the combatants try to annihilate each other opens doors to more varied styles of playing. Creatures and characters can parley or prisoners can be captured and their releases negotiated. The party can encounter creatures too powerful for them and be driven off, leaving unfinished business that the characters will want to return to once they have honed their combat skills.