Shinra is the most difficult among all the players. Getting 50 might seems impossible as Core Sphere 1 appears a lot more frequently, as much as 5 times in a row. If it happens too many times, you should really... restart. Even if it means, playing the third round of Sphere break again. You can defeat him by using 3, 4 coins each time, but still the core sphere 1 is very damaging. Need a lot of patience and luck for this one. If you are lucky, you can even add in 2 coin combos to win him.

Coughs*

And uh, you can save at the Blitz stadium after two of three defeats so it's not such a huge pain in the arse to restart ^^ Took me a long time to beat him so good luck ^^

Link: http://www.eyesonff.com/ff10-2/playtounffx2.shtml