You'll have wanted to beaten Amon, although it's not absolutely necessary.
Step 1: Ensure you have level 100-200 unlocked. Also ensure that you're already about lv 75+ before trying this. You need to be this level because you'll tend to die against normal monsters at level 101 easily. Also make sure you have a handful of paradise apple: body on you. Wouldn't hurt to have the other two paradise apples either though.
Step 2: Go to the Forest Thickets. You'll want to go to the rift areas of the map. The ones that have space as the background (a light-blue colour though). *note: make sure you have no sopias that have any type of physical counter on*
Step 3: Your goal here is to have an encounter with a cricket type enemy, and two other enemies of enemy type. Seriously, it'll be a huge waste of exp if you don't have three enemies right away when you encounter them. Any ways, once these conditions have been met, kill everything except one cricket type enemy. The reason you want at least one alive is because they have a special technique called "call for reinforcements". This brings one more cricket type enemy on to the battlefield, and it is worth full exp. Let's say these crickets are level 101, they'll give around 10k exp. Activate a paradise apple:body, and this turns into 20k exp. Kill one, then let a cricket use call for reinforcements. They use it very often, rarely not using it for 3+ turns.
To recap: Encounter cricket type enemies in the forest thickets, in the rift areas, make sure you don't have any type of counter on, and make sure there are 3 enemies when the battle starts. Use your paradise apple:bodies, plus others if you want to,(I suggest using the sp ones, as you'll get 173 sp at the end of every fight.) Kill all crickets, leave one alive, let it use "call for reinforcements, rinse and repeat untill you reach the max experience for a fight (999999 exp). I recommend having a calculator beside you to add up how much exp you get per kill.
Example: At the time this was written, I was lv 160 with both of my current characters (darwin and Orlando) and encountered the crickets, then did the steps above. Since crickets are worth 22.5k at my level, if I then use a paradise apple:body, they become worth 45k. Thus, I need to kill 22 crickets to obtain max exp for the battle. This is about 10 minutes for me, which is decreasing in time as I level, because the enemies become worth more exp. So, I get 999999 exp for 10 minutes of my time. As there are no experience Billikens in this game, this is the fastest way to get exp. If you also use sp apples, it'll be a major help for maxing all your Sophia's as well, although grinding the sp billikens would be faster, this way gets you exp and sp.
Head of the Benevolent Church of the 3-13 Archer. "I'm the stripper your husband hired." ~anonymous.