The Insidious Earling is a formidable opponent. His high damage output means it is best if you bring a party to defeat him. Though, it is possible to defeat him alone, it generally requires all the right equipment which means having high end cards / equipment which can cost a lot of zeny.

• The party should contain at least 1 assassin_cross or taekwon_master for melee attacks and at least 1 high_priest for support. Priest should use buffing skills, heal, and resurrection
• Use any element for attacking except poison or neutral. Since the opponent is a Ghost type, those two elements will do little/no damage. If using a taekwon_master, use warm_wind level 5 to enchant your attacks with ghost so they do more damage.
• Bring plenty of Reraise Potions and Yggdrasil Berries just in case.
• For melee characters, make sure you have over 500 hit as the mobs have a high amount of flee. You can gain more hit using Phreeoni Cards if you lack enough dex.
• After you defeat earling one time, kill the mobs before attacking him. If he dies before his minions, he will respawn with even more.