To quickly get your characters and their weapon levels up, go to a
stage that has a large amount of 'Invincibility' geopanels such as
Episode 5, Stage 3. In stages like that you can manipulate the enemies
onto the non-invincible geopannels and hammer away on them without
having to worry about taking any damage. Once the enemies have become
too weak to derive any real experience from, you can lift them and
toss one onto the other to combine their levels, making one high
leveled monster. Repeat the drill until your characters and weapon
levels are where you want them.

NOTE: You can level your weapons by attacking allies on the invincible
geo panels as well.