Oh yeah, you could try reflecting his binding attack so that you don't have to play as Riku, but I forget whether or not he'll just keep using it until he finally succeeds...
one more thing there is also a bonus dungen is that at the end there is a room where you can refight every single organization member and get either a boost for sora's stats or a rare syenthesis item
and a new difficulty level which a the starts gives you abilitys at the begginning and one of those abilitys allow you to obtain no exp which means that you can fight xemnas at level 1