There really wasn't anything hard about getting 100 percent, but if I were to say a few missions made it somewhat difficult to get an S rank on, since I wanted S rank everything. Overall, this was easy to 100 percent.
Yeah, I don't think the debris counts. You just have to avoid the boss.
For me it would have to be the "Can't Touch This" Achievement/Trophy
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Red Rings, already accomplished that, wasn't too terribly hard. and the acheivements thing says i haven't. I have I've checked from right off the game. stupid city escape and having to hit it perfectly. I'd say S ranking it all. still have yet to do so.