I won't lie; when I totally used a Sphere Break calculator to cheat when beating Shinra. I managed to beat the other opponents just fine, but for some reason I wasn't getting good core bonuses with Shinra until the round was almost up, so after a couple tries on this recent playthrough I got fed up and cheated.
I remember wanting to burn the BradyGames strategy guide because it outright misleads you about what you're supposed to do during the Mi'hen Highroad mission in Chapter 2. It makes it sound like you want to try your best at capturing the chocobo during the second section of Highroad when what you want to actually do is get it tired, then let it escape to the third section.
I missed out on what couldn't have even been half a percentage point there the first time I went through the game, and then had to rectify it the second time through while still fulfilling the other requirements for seeing the Perfect Ending on that playthrough -- since it's not a matter of percentage points alone, but also requires doing a few things during that specific playthrough (listening to everything Maechen has to say outside Youth League Headquarters in Chapter 1, as well as in Chateau Leblanc during Chapter 3; pressing X/hearing the whistle four times on the Farplane at the end of Chapter 3; getting the Episode Complete for Besaid and then Luca in Chapter 5; and finally hitting X while crossing the Farplane Glen once again at the end of Chapter 5, then answering "Yes" to the Bahamut Fayth's question).
I got the 100% first time through on PS3 but I remember how tedious it was on PS2 because all the guides are missing something. I think the major stumbling block for most people is the cameras in chapter 4, you have to repeat some of them for like 0.2% even though it looks like some of it hasn't changed.
My first playthrough on PS2 I got like 96% and never cared enough to try again (for 100% I mean, I played it again for the HD of course). I like the game, but eff the 100% ending. It's not even prerendered.