This method is fast, but also a little complicated. You need Eric to Know Critical Claim, which is in his Guile skill tree. It's very helpful if the Hero knows Pep Up from the Luminary skill tree. Jade's Lightning Thrust from her Spears skill tree is a little helpful. Give all your Pep pips to Sylvando. I went to the campsite outside the Dundrasil ruins.
1: Pick a fight with a Malicious Knight errant, using a party of Jade, Rab, Eric, and Sylvando.
2: Defend until they each get Pepped. Swap them for other party members to keep them pepped up. Should take about 5 minutes.
3: Once all of Jade, Rab, Eric, and Sylvando are pepped, use pep up on the Hero and end the fight.
4: At the campsite, rest and SAVE your game.
5: Start a fight with a Malicious Knight errant, using Hero, Jade, Eric, and Sylvando.
6: Have Hero, Jade, or Erik use Hallelujah. This gives an XP bonus and guarantees item drops.
7: Swap Eric to Rab
8: Hero uses Pep up, Sylvando uses a pep pip on Jade.
9: Hero, Jade, or Sylvando use Electro Light. This replaces the enemies in the fight with either robots or metal slimes. The metal slimes are worth lots of XP, but are only weak to critical attacks. If you get the robots, quit to dashboard and reload step 4. Try again.
10: No one attacks. Swap Sylvando for Eric. Rab uses Dirge of Dundrasil to put the metal slimes to sleep.
11: Jade uses lighting Thrust and Eric uses Critical Claim to kill the metal slimes.
This should give 3-5 levels to your entire team. At level 70+, the Metal slime king will drop another pep pip, replacing the one Sylvando uses.