It makes total sense, and it's understandable to be reluctant in this situation. You can always set a certain level range, and work off of that. Thomden did some wonderful bookmarks that operate in this way, seen here. Those bookmarks are excellent because they show how you can keep it short and sweet, but still convey the overall idea that you're trying to get across.

You can also throw level-based out the window and work purely off roleplaying. Like they say, there are many different kinds of monsters in the world, and a lot of them are not your level. "We have to fight until the end, until we're all dead if that's what it takes!" said no player ever, if the chance of running away is readily feasible. Maybe there is a baddy way beyond the level of the PC's in this dungeon, and is that unreasonable? Not at all, it will give the players something to work towards if anything else, and there is no harm or foul from the PCs waiting until they're a higher level and better equipped to handle something of that challenge rating.