Completing every level on every difficulty will give you ~1,800,000 experience points, so plan for an extensive grind.
The most profitable level is Order 20 -B- Hard but it's four boss battles, which are very, very hard and tedious without some tricks. This level gives you ~43000 EXP on SS completion. That is, for the last 3kk EXP you'll need to grind it 75 times. Good luck!
The one before it, Order 20 -A- Hard, gives 19000, two times less, but it's a lot simpler and it's possible you'll be able to complete -A- more than twice in the time to complete the -B-.
Manpukumaru at lvl 50 one-shots the downed Gundari with her ultimate attack, by the way...