delericho
Legend
(How do you clue players in that...) "You can't survive this monster"?
I don't. I give hints that a particular creature is fairly badass (for different values of badass, of course), by having it referred to be the locals, by showing signs of the devastation it causes, and by giving PCs opportunities to research their opponents, but not any indication of how the monster relates to them specifically. It's up to them to do the calculation of whether to risk it, or whether discretion is the better part of valour.
Players are notorious for getting into fights, and fighting until a bitter TPK.
That's true... but that's their call. It's not my place to make those decisions for them.
Most encounters with monsters are written to be overcome in a glory of blood and battle, so players naturally and quite reasonably assume that any monster they come across is killable.
I've generally been lucky in that I've played mostly with people used to fairly deadly (but not absurd) games in the past, so they know that such an assumption would be faulty. However, if I did find myself with a group mostly composed of "if we see it, we can kill it" players then I would handle the situation by stating clearly that my campaign doesn't work like that, once at the outset of the campaign.
If they don't take that warning on board, or choose not to believe me, then I'm afraid they get to learn the lesson the hard way.