I have achieved 100% before. I have achieved 100% in one game before. I had a guide, but I didn't really find it that difficult. There are maybe a handful of things that you probably wouldn't be able to figure out by yourself before you messed them up. Everything else is really just visiting every area in every chapter and doing every mission the best that you can. And watching the commspheres. It wasn't all that bad.

As easy way to level up is to go to the New Cave in the Thunder Plains. Have all the girls be Lady Lucks, and teach them Double EXP as soon as possible. The will level up down there in no time. For a faster approach, try equipping a Lure Bracer.