Put Hero, Jade, Sylvando and Rab in your party. Have all at full pep. Start battle. Have Hero/Jade/Sylvando cast the triple pep power "electro light". Will replace all enemies on the field with various metal slimes. If you luck up and get a King variant, use Rab and use pep power Dirge of Dundrasil, which guarantees it goes to sleep. In lieu of Rab you can also use Erik and Critical Claim ability to insta-kill one of the slimes. You know what to do from there.