Level 20 Characters:
     First,   this   code  only  works  in  the  females  story(to  my
knowledge).  In chapter 2 beat all the enemies so only Zakuro is left.
Then  corner  him.  He will just keep creating phantom miles. Have one
character keep attacking the phantoms until they are at level 20. This
is also good for getting the secret phantom items. It is easier if you
make a clear game save from Toxas quest, so you can use the weapons.

Level 20 easy with Toxa:
     In  chapter  7  (when  you  fight  Cross) defeat all the Phantoms
except  the  ones  near  Cross.  When all the other Phantoms are dead,
start  attacking  the  ones  near Cross, but don't kill Cross himself.
Once  you  kill  off  enough  Phantoms, Cross will just keep on making
them.  Stand  just near Cross's move limit and wait there. When the he
creates  the  Phantoms they will just move up to attack you. Kill them
off  and  continue waiting. Cross will do this forever. Level all your
people up to 20 and then kill Cross.

Level 99 Character:
     Use the female story, beat the game and use the male and beat it,
play  the  female  again but for the first 2 stage only, then play the
male  story  after you save the game and then you will notice that the
characters are very strong. Level 99.