After much experience with the challenges, I concluded that the LVL 1 Run was the most difficult to get through, and it can be made even more complicated, if not impossible, by restricting item usage, magic, or abilities. The LVL one run is distinctly more difficult in the first game because it wasn't designed for that, even though they added the NO EXP ability later. Kingdom Hearts 2 & 3 are much easier games for EXP Zero.