This is selectable from the beginning and really is not too difficult with the exception of maybe having to replay a couple of the boss fights. All the strategies I have in the guide are for the “Challenging” difficulty. You can revive your partner if they get KO'ed and if you both die, you don't have to do much backtracking. The difficulty achievements are stackable so beating the game on hard will also net you “Squad Initiate” (easy) and “Squad Member” (normal). I’d played through solo but it may be helpful to have another person play too because the AI sucks.