DQM 3 Boss Help, Please!

I'm at the later midgame and I keep losing poorly to >!Whiteworl the Watcher!< in the middle area of The Cradle of Corruption. I've been using a Metal Slime Knight for group healing, the Cane Custodian for physical attacks, and a Merman for elemental attacks and backup healing. My benched team fluctuates. I've tried leveling up, but it's to the point where its taking a while as my team is in their high 30s-early 40s in levels. Is there any level-up trick or specific synthesis I should try? Thank you!