What do you consider 100% completion? Let's take Borderlands 2, for example: does 100% entail getting to max level with all six characters? Clearing all the DLC with all six? Clearing every single side mission, so you don't miss a single line? The thought of it is overwhelming and sounds horribly repetitive, since you have to do everything six times over.
I never 100% any game. I got very close in Final Fantasy IV PSP, doing the Lunar Ruins and ending up with every single party member at level 99, but I didn't beat Brachioraidos, because the way to beat him is tedious and stupid. I think you get a special shield that's essentially useless, because he's the hardest superboss. I gave it a couple of shots and I just didn't wanna go step-by-step through a walkthrough.