How to get over 200,000 Mona points or EXP per battle
Mona points can be useful in raising the rest of your characters closer to level 99 if you don't want to battle them. For this, you'll need a character with powerful spam-worthy AoE skills. Adol and Chester work great, as I'm sure do many others.
Pipiro's Cap (for Mona points)
Pockle's Tail (for EXP)
Bandit's Mask IV
Clown's Mask IV
(Only 9 AP!)
These three will double your money, and double your experience and then convert that into more money. For the battle, you'll want a 1v3 setup, preferably on Nightmare, and make sure the character you plan on using is level 99 too!
Your Support character will have to be someone that can assist heavy spamming - Kevin or Ries work great. Support Book of Enlightenment will also allow you to summon them twice if you need to (but usually, one good combo is all it will take) and a Silver Crown is great to start it off from the beginning of battle. Also consider a Platinum Crown or Spear of The Way if the Nightmare NPCs make quick work of you.
In Free mode, set your character to have a 5-star handicap, then select 3 NPCs to be on the same team, all short-range characters (that is, ones that will run up right next to you) and set their handicap to 1-star. You don't absolutely have to, but it makes it easier, and on Nightmare mode you can turn into shredded meat pretty quick in a 1v3.
My strategy is to activate my support right off the bat (it was Ries with Adol and Kevin with Chester) and alternatively spam Spinning Slash/Spinning Chaos Dance and Avenger/Demon Thrust. There's usually a pair of high-SP spammable skills with each character. For instance, Cocytos/Diamond Dust with Klose, Arc Drain/Kienzan with Leowe, or Brave Smash/Crimson Ray with Lloyd. Keep in mind, however, using the same skill repeatedly in the same combo will greatly diminish its power - thus, for optimum damage, I would only use each skill roughly twice, then retreat and wait for the combo counter to reset, then do it again.
It takes a small amount of practice but you should be able to clear a 1v3 battle in this way in about 15 seconds, and it will net you 200,000-250,000 mona points, roughly, or EXP, which is great with the Adventure Diary accessory. Yay! You can level a mid-level char up to 99 in a short time with this trick!
If you have trouble with Nightmare, turn it down to Normal. You'll only gain about 100-150k Mona points, but that's still enough (with Pockle's Tail) to get 1 character 5 entire levels every battle all the way up to 99!