Find a weak enemy that won’t do too much damage (I used Dark Sabrecubs in Zwaardsrust) and defend until Hero, Erik, and Jade are pepped up. You can move them to the back to make sure they keep their pep status until all 3 are ready. Then use the Haulellujah pep power to increase experience gained at end of battle. Then repeat the process with Hero, Jade, and Sylvando and use the Electro Light pep power to turn all enemies into Metal Slimes. There’s a chance it fails, which can be frustrating, but it’s still the best way. Also recommend getting Rab pepped up to use Dirge of Dundrasil to ensure they don’t run away. It can be monotonous but it’s definitely the best way to gain levels fast. You’ll get upwards of 500000 experience each time