to get the "best" ending then yes you need to do at least some of them.
But to finish the game you dont need to do any of them I dont think? They take some getting used to but they arent that hard, if you struggle, then put it on recruit to do the "best" run through of the game
They each have an achievement for beating them, but to get the difficulty specific achievements, and the just beat the campaign one, no you don't need to do them. Don't even bother trying to beat them on anything higher than regular difficulty either. --- I'm careless, but I care. GT: Noitrez