So it's more when you get to the 30's then?
Late to the response party but no. Assuming you're rolling on Advanced mode (which you have to manually toggle on by the way-- don't forget), each challenge map kinda jumps up 5 levels or so in terms of par level. You can totally beat them without grinding RIGHT UP to that point, but you should at least play up to the point where you can field the necessary manpower.