Oddly enough, they were the easiest ones to get an A with, for me at least.
If you're up to it, I can try to help you get an A rank.
Pierce and Ada were easily my hardest. Helena was pretty hard too, but I did her with a very good friend. All three of them suffer because their guns are not reliable headshot staggers, so they can't pop+melee everything, and they burn through pistol ammo like whoa.
Pokemon White Friendcode - 2193-7011-1635 BEEP BEEP RICHIE!