In the fire of mount doom is a secret area, where you can find a spot with 20 omegawyrm’s.
they easly give 15k experience a battle.
Get monsters that have an attack that hits all three opponents. If you have unlocked crescent island, go to Deucalis. Then go south to the nearest beach. You will find anywhere from 7 to 27 Biteschools. You can get almost 9000 for 25 or so. Oh, your monsters have to be considerably leveled up already (about lv 60) to kill three every shot. Good luck!!!
10 to 30 Biteschools, the number in the top right corner is the number of reinforcements, then there’s the three already on the screen
The locations spikes mentioned is the best place indeed