If it's unbeatable, have it end soon after it starts. If people don't know better they will waste their items trying to win and that's the cheap part I don't like about unbeatable battles (though I mean I think it was FFV? One of the battles was unbeatable and I knew it but I decided anyway to put up a fight because the game let me but that doesn't work all the time). I had one in one of my games and first thing he does is wipe you out but that was the point. Since you were supposed to be made to believe hey this is my first boss in an otherwise generic rpg outing! Awesome! And then bam dead player and then the story shifts.