If you want your players to run from a fight, do not make it their choice. You have designed a fight that they can not win and expect them to know they can not win when it is impossible for them to learn that information unless you tell them thus defeating the point of making the fight in the first place. So unless you’re okay with killing members of the party just so they can run away, the only way to ensure they will leave is if you don’t make it their decision.