Ok why cant I go to where I fought young xehanort and get a rematch? And should I beat julius then do the boss portals? Ans how much should I grind for julius?

1. You have to beat Sora's boss 1st?

 

2. Your choice.

 

3. Just use balloonra/ga.

You can only fight Young Xehanort after you beat Xemnas with Sora. That goes for every time you want to fight him. Put simply: If you're playing as Riku and you see a drop gauge, then you need to fight Xemnas before you can fight him.

1. probably save before you fight him.

2. depends on what your level is and what you feel like accomplishing first.

3. use balloonga and balloonara to beat him.

Balloons X_X Wierd yet powerful

i would suggest do secret portals first so you can use their hp boosts for julius unless ur up for a challenge!

I would leave Julius to last with the portals if you can (I know there's a normal portal in the same area that if you haven't beat before you cannot finish it til Jullius is done)

1 - Xemnas if you may.

2 - Your choice.

3 - Balloonra