Yu can level to 50 without doing Dungeons.. But you still have to group.. It is called FATE parties or FATE grinding.. The problem is if you do not do the story quest and the story dungeons you will not be able to experience or enjoy the end game content. Cause you must complete all the storyline quest and dungeons to unlock the hard mode primals and the Binding Coil of Bahamut. So it is your choice of course if you wish to just hit 50 and then do a bunch of low level stuff to be able to play the whole game.. Have and enjoy your time