right the real way to do it is this 
it may take a bit of time but gather all the ingrediants to make the ultima blade ( this
will definatly increase your power)
You must use all pp ups on sora as magic will come in handy
Train to at least lv 60 but i would say lv 70 if you want to be safe
As the unredgistered said carry potions ( max potions would be better & at least 2
elixers)
Make sure you know lv 3 high jump & lv 3 glide, double jump, slide defend etc.

how to beat him
To start of with just defend - ( not sure if you have played it but in sonic and the black
knight when it comes to fight shadow again all you do is block at first)
Anyway - after you think you've defended enough ( i say when u think coz i cant really
know how gd u are can i :P) 
go for the attack keep on repeating this until he does another move - explained below

HEARTLESS ANGEL ( HIS SPECIAL MOVE )
This is where the ultimate blade comes in handy, when he is up in the air do lv 3 high
jump to get to him - do enough damage to him ( 1 HIT WITH ULTIMA BLADE SHOULD DO IT) He
will fall back down to the groung

BARIAL FLAMES 

when he does this defending wont be enough you must run away right to the edge and the
glide to each side of the stadium ( for safety coz u dont know how far it will go 

METEORS 

for this just roll and dodge coz thats all you can do there is no hitting them back at him
or stuff you must just dodge and run

DONT WORRY IF HEARTLESS ANGEL HITS YOU
just use a max potion coz that will boost your life up a bit coz it takes you down to 1 hp

ALSO ON HEARTLESS ANGEL 
if it hits u dont use cure or potions yet coz its not over yet he will start attck ing you
straight away so as soon as you get down to the ground he will attack so be ready to
defend 

AFTER DEFENDING ONCE HE HAS DONE NORMAL ATTACKS
after you have defended, you should be able to get about 4-8 attacks in but then get ready
to defend again

HOPE THIS HELPS :p