You have to remember that a clean save was only needed pre-patch. Now you can do it without deleting your save game from the memory card.
Just replay the chapters getting your A/A+ and you will get the achievement, at least other people have been since the patch even on a used save game. As the chests now work so should this.
I would also say that even if you once obtained an A/A+, if you redo a level and get a lower value that will be saved instead. Remember that like the Treasure Hunter you must finish on the last boss CH15 and watch the credits to unlock this achievement. Also I would not risk doing it on a staggered play CH4, CH8, CH1, although technically it should work I wouldn't risk it.