At this point we're crossing whether it's a problem with the scenario or a problem with player attitudes. If you're losing a fight, RUN.
Everyone time this sort of thing is discussed, someone brings up the idea that players should just try to flee encounters that are too difficult for them. When this happens I wonder what the practical experience has been of this theory?
I've been running various editions of D&D for 30 years now, and in no edition of D&D was this ever a particularly practical solution.
1) In general, by the time you realize you are in over your head, it's too late to flee. This was true even in 1e AD&D where a generous subsystem existed for letting you evade monsters abstractly. The problem was getting the whole party disengaged successfully so you could utilize the subsystem was difficult. One issue is that in 1e A&D you took a proto 'attack of opportunity' if you tried to withdraw from melee. Worse, this attack was made against the character's back! If you wanted to actually survive 1e combat there were two basic rules: a) don't get surprised, and b) don't expose your back to the enemy. If you want to succeed in fleeing, you have to flee before you are engaged. But this decision is counter productive in most situations unless you have an easily recognized marker of difficulty.
But even more to the point, generally the realization that you need to flee occurs when the party suddenly finds its either about to take or has taken casualties. PC's take massive damage and someone is now either dead or will likely be on the monster's next action. In most D&D systems, combat only lasts 2-4 rounds on average. If you figure out even as early as round 1 that you are in trouble, it's probably too late to rationally flee. If you abandon the body to the monster, you probably can't raise the character. If you try to disengage, you are giving the monster at least 1 free round of attacks. Ninety percent of the time it is a vastly more rational strategy to risk the 1 round trying to kill the monster over granting the monster 1 free round while you try to disengage. This number can go up to 100% of the time depending on the DM.
2) Without an evasion subsystem, as in say 3e, you can't actually run from anything faster than you are. And most monsters are faster than the party, often by a good deal. So literally running away is often a non-strategy that will be countered and made into a useless throwing away of your action, unless you have a win button like sufficient Teleport prowess to whish away the whole party. Otherwise, you are far better off generally in every D&D system of turtling up, trying to get the monster to engage a different PC, or spending some sort of win button resource you've stock piled for this eventuality. And in general, if you are faster than the opponent, you probably also have been missile weapons and the best strategy is kiting it rather than fleeing it.
3) In practice, running away at your best possible speed is de facto splitting the party. This is called 'a rout', and as in real life, the moment a military unit routs is the moment it is most vulnerable. In ancient warfare, the vast majority of casualties that a force would endure usually occurred during the rout, when the defeated side was trying to run away, unit cohesion had broken down, individuals could no longer rely on the mutual defense of formations, and ones back was toward the weapons of the enemy. As a DM, many of the worst mass party deaths I've inflicted on the PC party occurred precisely because the group decided to run rather than stand and fight. Running in no way guarantees the monster will be less dangerous.
I think some DMs actually play with a house rule that is something like, "Since this encounter is unbalanced, if the PC's flee, then they should be rewarded with success." But this house rule is less than obvious unless you've experienced it.
