After you're able to defeat the Jargons in the Maella Region in a couple of hits and the Dingalings only cause 1 to 2 HP of damage to you, you'll be able to get as much XP as you could possibly want.

All you have to do is run around (or Whistle) until you run into a Dingaling. Do not kill the Dingaling, you should kill all other monsters that assist a Dingaling and leave only one Dingaling on the field. Have Jessica, Hero, and Angelo (if applicable) defend and Yangus Psyche up.

When the Dingaling calls for backup, attack with Yangus for an easy kill. Keep doing this and the Dingaling will keep calling Jargons to help it, and you can kill them in one move. If the Dingaling calls for backup before Yangus psyches up enough, just have your weakest attacker assist him.