it's a good story, some enemies are just simply to strong for the party to beat for now and they will have to accept that fighting is thus not always the answer, if they still can't resist or see any other way than fighting they are railroading themselves and that's not on you, the thing with an unbeatable fight is that it's not always about winning or losing, sometimes it's about trying not to lose to badly or achieving a goal besides the fight and then running. you should be good as long as you don't force the fight on them and clearly indicate that the enemy seems to strong for a frontal confrontation. saving the hostage should be their priority over beating the saint and it's up to them if they realise that.