Recommended difficulty for campaign achievements?

I find Campaign's achievements (bonus objective) to be the most fun bits of the campaign. At the same time, I once heard that some of the achievements are straight-up impossible to get on brutal difficulties, yet I find them to be the most fun bit of the campaign! I am curious to know your tips.

TLDR: This thread serves as the forum for discussing recommended campaign difficulties for completing achievements, aka bonus objectives.

It helps to focus on time-critical bonus missions first or to do them on a different run than the others. I never tried the campaign on a lower difficulty than brutal. If things are going too fast, it also helps to pause the game or play on a lower speed sometimes.