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sweatdrop You dont buy Omnislash and W-Summon with GP. You get them with BP. BP is earned through the Battle Arena in the Golden Saucer.

It's kind of a waste to save up for omislash since you get it in the battle arena. Your bps go away as soon as you lose so make sure you stock up on high potions and just try not to lose.

Two words "Chocobo races"
Get the gp rather than the items, soon enough you'll have more bp than you can spend.

As for the actual fighting, that's the easiest thing I've done in a while.

Go back to the chocobo ranch, and buy some stalls, then send a few chocobos to the ranch. Put your favorite in a stall. Go buy some of the most expensive greens from the choco sage, and feed them to your chocobo.
Go back to the gold saucer, and in the racing arena go to the back and talk to ester, shell let you register, and run the race with your chocobo. If you have tifa in your party, sometimes she'll ask to ride for you.

thats a pretty sweet idea

*grumbles* now i have to make a golden chocobo cause S rank cant be beaten by any of my wonderful chocobos