This is a weird thing. I played through with one of my sons and he would get most as we would play through but not all esp if it was something specific for only one person. But, I played through on the free play w/my other boy after I had beaten the game and almost every single achmnt popped after just that one level!!!!
It was crazy! He has more achmnts for the game than my other boy who played through every level w/me.
So I have no idea how that works. I reloaded the last level and played free my son who beat the game originally w/me and he didn't get any new ones. Crazy