Usually, you have to defeat a boss in an RPG to move on
But, sometimes, you are suppose to lose the fight. These fights are there for story, and also to show that the heroes really can't defeat the bad guy, because, well... The player tried their best, and couldn't win...
So, those of you who had those kind of fights in their games. How did you set them up? How did you insure that winning is imposible? How do you make sure the hero doesn't waste all their items?
How do you make sure the player doesn't just reset the game it they see that they can not win the fight? I mean, in games made with pc rpgmakers, you can just press F12 to return to the title screen. So how can you ensure that the player will acually lose the fight, and not try to win and reset the game every time?

I've seen a lot of this over the years in commercial RPGs myself. The worst way I've seen it executed is where the boss has normal attacks and defense and such, but has infinite hit points and you just don't know it. Then you wind up wasting tons of time and resources, and maybe even restart a few times in frustration. The BEST way I've seen it handled (Xenosaga ep. 1 comes to mind) is where you actually CAN win the fight if you go all-out; the plot progresses the same way, there's just slightly different dialogue and you maybe get a nice XP, money, or (non-missable but still advanced) item as a reward. I've been toying with the idea of hopeless battles for the game I'm working on, but haven't come to a decision yet. I think that if you were going to do it, the best way might be to make sure that the battle doesn't drag on. No more than a handful of rounds, and maybe the boss has some dialogue or something that makes it perfectly clear that it's not a standard battle situation.

~Rosser

My game has a couple fights you're supposed to lose.
The first instance, there's two things that clue you in. One is that the song that's playing is not the boss theme, instead being kinda sinister. Two is that every time the enemy attacks, he says "stand down".
The second instance, the boss has two phases. The first you're allowed to win, the second has the boss get a second wind where he uses a more powerful attack, and again, the sinister music plays.
And of course in both cases, their HP is maxed out.

That said, these precautions aren't foolproof. I watched someone deal with the first fight, and despite the situation, he still used up all his items.

This is one of the few RPG tropes that I like seeing. I don't know why, I just always got a kick out of these situations. Namely I like replaying games and seeing how well I can do against them.

One of the best examples off the top of my head that show off this kind of thing done right (which ironically negates my reasoning above) is Legend of Dragoon.

For your question, Survival II does have one of these, but I won't provide context as the game's still not out.

Some of the best ones are the ones where you're meant to lose but can still technically win. Sure, the victory doesn't count for story reasons (most of the time) but in some games you'll get a nice bonus for the win.

Some examples:
Lufia II and The Legend Returns: Bonus equipment.
Final Fantasy IV: No change but the boss can be beaten.
Tales of Destiny: Avert the entire plot and end the game right there!

I really like this trope. I especially like when the player is supposed to lose a battle and yet it's still possible to win, and you get some sort of reward or alternate dialog out of it.

I only example I can really think of this is the first fight with Gades in Lufia 2. If you manage to grind to some pretty high levels before the fight to manage to actually beat him, you're rewarded with the Gades Blade and instead of leaving the battle with everyone wiped out, he instead uses an energy blast to wipe out your party after the fight is over.

I haven't played many RPGs, but the ones I remember are from Defenders Of Oasis (Game Gear) and Chrono Trigger (SNES).

In Defenders Of Oasis, the very first boss character you have to fight is technically unbeatable. He wields a special sword that causes his target to black out (unconscious). The fight literally goes like this: My guy hits him, he uses his sword, and it's all over. Somehow, the hero is rescued off-screen and wakes up among the underground resistance that the locals have formed to fight back against the invading bad guys. He then learns that there's a genie in the castle who has a spell that protects from the unconsciousness-causing sword. You get the genie, who joins your party, and then defeat the bad guy that way.

I don't really have any feelings about that, one way or the other. I s'pose it was kinda cool at the time, and it served the story because that's how you learn about an important character (the genie). But from a game-play standpoint I don't really understand the appeal of engaging in a losing battle. I think that kinda thing could just as easily be handled with a story scene, especially in today's story-heavy games. I could take it or leave it, as long as it's obvious from the damage I'm taking or some other cue (as in Defenders Of Oasis) that it's a scripted event.

I'd imagine it's so that the player can feel that sense of hopelessness, rather than simply see it in a cutscene. And in some cases, it's for the shock value of "wait, I have to fight HIM?"
For example, in Final Fantasy

"Unwinnable" (or practically unwinnable) battles aren't something I've ever used in my own games, but I've usually found them to be pretty frustrating. They can be great storytelling devices if done right, but I feel like the developer should do everything they can to not make the player feel cheated of either their time and effort, or their items.

* Scripting in character dialogue to notice that nothing is working and the enemy is too powerful is a nice hint to the player, and can enhance the storytelling power that was the sole purpose for the battle in the first place.

* Giving the enemy stronger stats every turn, or scripting them to unleash more and more powerful attacks on your party each turn, is a good way to ensure the player doesn't waste too much time in a battle that seems winnable but definitely isn't. The idea is that if you're pretty powerful, but not powerful enough to actually win, you'll get overwhelmed quickly instead of dying in a 60-minute battle of attrition. This can also be tied in with cool battle events and dialogue.

* If a concern is that the player will be pissed about wasting a lot of their good items, a solution is to have a treasure chest somewhere soon after the event that happens to have exactly the items that you used in the unwinnable battle (this can even be done in XP/VX/Ace with Ruby scripting).

* Short of doing that, a game could reward the player even if they didn't win the battle, based on how much damage they were able to do to the "unbeatable" enemy. An NPC who compliments you for putting up such a good fight, then rewards you with a nice item or collectible, would really take the sting out of realizing you were Sisyphus.