hard?, I think it is a bit if you are playing with both characters at first
it does get easier, dont wanna spoil you though
but if you dont care:
spoiler (click to show/hide)
joshua is clearly the easiest to match with and get those partner combo things. Basically you can press X then mash Y or B which spams the really fast (but really weak) laser looking attacks, which give insanely fast powerups. then you can focus fighting with neku, looking up just to see if joshua has been knocked down once in a while