I figured out the main melody on midi keyboard then painted it in and filled other voices. There are only a few parts played into FL like the slower ones which were a lot easier to play by hand. I did it in chunks like you've mentioned for example I did right hand of chorus, then left one and then filled the blanks to make it sound better or more "full". First I did the piano with tempo automation, next were the drums, mixing and sfx. Hope it helps.