5-2 works for those who can hit a 2x2 area (basically just spells, Giant Desco, and a few Giant Magichange effects). 8 Stronger Enemy Bills makes them all level 99 and you can toss the yellow Geo Block in the lower right onto the stage and destroy it for a quick +150% EXP bonus.
Next is 9-4 but the lay out makes it awkward for anyone to clear in one shot (without throws) besides Giant Desco. 1 Stronger Enemy Bill makes them level 99 or you can max it out to get them to level 445.
Alternatively you could capture level 99 enemies from 5-2 or 9-4 and make a custom map with the enemies in a more convenient formation on EXP + 70% panels. I captured 13 of the Eryngi from 9-4 and put them in the formation of a Giant Big Bang. I used that map until I got to Mt. Ordeal 4.
"Recompiling. Shifting battle modes." - What Raki is supposed to say when she crashes your game. - Ar tonelico 2: Melody of Metafalica.