I beat FFII with Level 55 characters.
You don't level up in FFII. >_>;

Now, to compare...

FFI: Mediocre. Not impossible, not simple.

FFII: Abusable system.

FFIII: Only truly difficult one.

FFIV: Mediocre. Some difficult moments.

FFV: Same. AP Goldmines don't help, however...

FFVI: Abusable everything.

FFVII: Mediocre.

FFVIII: Abusable.

FFIX: User-friendly, not too difficult.

FFX: Few hard battles, but abusable system, not to mention AP Goldmines. Not hard by any means.

FFX-2: You must not be pressing 'X' enough.