Oh wait, I read that wrong. I thought you said you owned the game! Definately beat the game before trying to get 100% since that takes a pretty significant chunk of time, probably close to the amount of time it took to get to the final boss from the beginning of the game.

But I suggest starting by leveling to 99 and taking all your forms to level 7, it will score you lots of items and you'll have the abilities and sheer strength to tackle some of the tougher 100% requirements.