I beat Be Objective 2 as a rank 4(barely), but it also happens in campaign. Once I hit rank 3 they got extremely hard, and specifically in Be Objective and the final Tower Defense they would leave turrets that make it nearly impossible to get by, so I just tried to see if it's any easier and it definitely was, I just got the achievement for beating the 3 star of all the challenges. Tower Defense 3 is still kinda hard at the end, because there ARE heavies here regardless, but they don't make turrets and shit like before.
The rest are extremely easy when you're rank 1. They really should patch the AI to make their aim not so insanely good, that's the main difference that happens. Then when you die they set up all their abilities, just making it worse.